Clean up configure.ac

Remove the obsolete autoconf macros:
AC_C_CONST, AC_HEADER_STDC, AC_TYPE_SIZE_T, AC_FUNC_STRTOD.

Clean up comments and whitespace.

configure.ac

@@ -1,6 +1,6 @@
 # configure.ac
 
-# Copyright (c) 2018-2019 Cosmin Truta
+# Copyright (c) 2018-2022 Cosmin Truta
 # Copyright (c) 2004-2016 Glenn Randers-Pehrson
 
 # This code is released under the libpng license.
@@ -89,9 +89,9 @@ fi
 DFNCPP="$CPP"
 AC_SUBST(DFNCPP)
 
-# -Werror cannot be passed to GCC in CFLAGS because configure will fail (it
+# -Werror cannot be passed to GCC in CFLAGS because configure will fail
-# checks the compiler with a program that generates a warning), add the
+# (it checks the compiler with a program that generates a warning).
-# following option to deal with this
+# Add the following option to deal with this:
 AC_ARG_VAR(PNG_COPTS,
    [additional flags for the C compiler, use this for options that would]
    [cause configure itself to fail])
@@ -114,10 +114,10 @@ AC_ARG_ENABLE(werror,
       CFLAGS="$sav_CFLAGS"
     fi],)
 
-# For GCC 5 the default mode for C is -std=gnu11 instead of -std=gnu89
+# For GCC 5 the default mode for C is -std=gnu11 instead of -std=gnu89.
-# In pngpriv.h we request just the POSIX 1003.1 and C89 APIs by defining _POSIX_SOURCE to 1
+# In pngpriv.h we request just the POSIX 1003.1 and C89 APIs by defining
-# This is incompatible with the new default mode, so we test for that and force the 
+# _POSIX_SOURCE to 1. This is incompatible with the new default mode, so
-# "-std=c89" compiler option:
+# we test for that and force the "-std=c89" compiler option:
 AC_MSG_CHECKING([if we need to force back C standard to C89])
 AC_COMPILE_IFELSE(
    [AC_LANG_PROGRAM([
@@ -133,21 +133,15 @@ AC_COMPILE_IFELSE(
    CFLAGS="$CFLAGS -std=c89"
 ])
 
-# Checks for header files.
+# Checks for structures and compiler characteristics.
-AC_HEADER_STDC
-
-# Checks for typedefs, structures, and compiler characteristics.
-AC_C_CONST
-AC_TYPE_SIZE_T
 AC_STRUCT_TM
 AC_C_RESTRICT
 
 # Checks for library functions.
-AC_FUNC_STRTOD
 AC_CHECK_FUNCS([pow], , AC_CHECK_LIB(m, pow, , AC_MSG_ERROR(cannot find pow)))
 
-# Some later POSIX 1003.1 functions are required for test programs, failure here
+# Some later POSIX 1003.1 functions are required for test programs, failure
-# is soft (the corresponding test program is not built).
+# here is soft (the corresponding test program is not built).
 AC_CHECK_FUNC([clock_gettime], , [AC_MSG_WARN([not building timepng])])
 AM_CONDITIONAL([HAVE_CLOCK_GETTIME], [test "$ac_cv_func_clock_gettime" = "yes"])
 
@@ -296,7 +290,7 @@ AC_ARG_ENABLE([unversioned-libpng-config],
 AM_CONDITIONAL([DO_INSTALL_LIBPNG_CONFIG],
    [test "$enable_unversioned_libpng_config" != "no"])
 
-# HOST SPECIFIC OPTIONS
+# HOST-SPECIFIC OPTIONS
 # =====================
 #
 # DEFAULT
@@ -383,9 +377,9 @@ AC_ARG_ENABLE([arm-neon],
          AC_MSG_ERROR([--enable-arm-neon=${enable_arm_neon}: invalid value])
    esac])
 
-# Add ARM specific files to all builds where the host_cpu is arm ('arm*') or
+# Add ARM-specific files to all builds where $host_cpu is arm ('arm*') or
-# where ARM optimizations were explicitly requested (this allows a fallback if a
+# where ARM optimizations were explicitly requested (this allows a fallback
-# future host CPU does not match 'arm*')
+# if a future host CPU does not match 'arm*')
 
 AM_CONDITIONAL([PNG_ARM_NEON],
    [test "$enable_arm_neon" != 'no' &&
@@ -395,7 +389,7 @@ AM_CONDITIONAL([PNG_ARM_NEON],
     esac])
 
 # MIPS
-# ===
+# ====
 #
 # MIPS MSA (SIMD) support.
 
@@ -429,9 +423,9 @@ AC_ARG_ENABLE([mips-msa],
          AC_MSG_ERROR([--enable-mips-msa=${enable_mips_msa}: invalid value])
    esac])
 
-# Add MIPS specific files to all builds where the host_cpu is mips ('mips*') or
+# Add MIPS-specific files to all builds where $host_cpu is mips ('mips*') or
-# where MIPS optimizations were explicitly requested (this allows a fallback if a
+# where MIPS optimizations were explicitly requested (this allows a fallback
-# future host CPU does not match 'mips*')
+# if a future host CPU does not match 'mips*')
 
 AM_CONDITIONAL([PNG_MIPS_MSA],
    [test "$enable_mips_msa" != 'no' &&
@@ -464,9 +458,9 @@ AC_ARG_ENABLE([intel-sse],
          AC_MSG_ERROR([--enable-intel-sse=${enable_intel_sse}: invalid value])
    esac])
 
-# Add Intel specific files to all builds where the host_cpu is Intel ('x86*')
+# Add Intel-specific files to all builds where $host_cpu is Intel ('x86*') or
-# or where Intel optimizations were explicitly requested (this allows a
+# where Intel optimizations were explicitly requested (this allows a fallback
-# fallback if a future host CPU does not match 'x86*')
+# if a future host CPU does not match 'x86*')
 AM_CONDITIONAL([PNG_INTEL_SSE],
    [test "$enable_intel_sse" != 'no' &&
     case "$host_cpu" in
@@ -475,7 +469,7 @@ AM_CONDITIONAL([PNG_INTEL_SSE],
     esac])
 
 # PowerPC
-# ===
+# =======
 #
 # PowerPC VSX (SIMD) support.
 
@@ -491,7 +485,7 @@ AS_HELP_STRING([[[--enable-powerpc-vsx]]],
          # disable the default enabling on __ppc64__ systems:
          AC_DEFINE([PNG_POWERPC_VSX_OPT], [0],
                    [Disable POWERPC VSX optimizations])
-         # Prevent inclusion of the platform specific files below:
+         # Prevent inclusion of the platform-specific files below:
          enable_powerpc_vsx=no ;;
       check)
          AC_DEFINE([PNG_POWERPC_VSX_CHECK_SUPPORTED], [],
@@ -511,9 +505,9 @@ AS_HELP_STRING([[[--enable-powerpc-vsx]]],
          AC_MSG_ERROR([--enable-powerpc-vsx=${enable_powerpc_vsx}: invalid value])
    esac])
 
-# Add PowerPC specific files to all builds where the host_cpu is powerpc('powerpc*') or
+# Add PowerPC-specific files to all builds where $host_cpu is powerpc('powerpc*')
-# where POWERPC optimizations were explicitly requested (this allows a fallback if a
+# or where PowerPC optimizations were explicitly requested (this allows a fallback
-# future host CPU does not match 'powerpc*')
+# if a future host CPU does not match 'powerpc*')
 
 AM_CONDITIONAL([PNG_POWERPC_VSX],
    [test "$enable_powerpc_vsx" != 'no' &&
@@ -521,7 +515,6 @@ AM_CONDITIONAL([PNG_POWERPC_VSX],
       powerpc*|ppc64*) : ;;
     esac])
 
-
 AC_MSG_NOTICE([[Extra options for compiler: $PNG_COPTS]])
 
 # Config files, substituting as above