API usage: add 'basic' configuration

This adds a new configuration file, 'contrib/conftest/basic.dfa' which
disables unused APIs on a test Linux-based system.  So support the
configuration several fixes were necessary in the test programs so that
the tests are skipped correctly when APIs are not available.

The configuration has been tested on a range of common Linux apps
including web browser code (qtwebengine), image processing code (e.g.
ImageMagick) and general display code (X11, Qt5 and Qt6, KDE).  Overall
this first step reduces libpng linked code and data size to about 2/3 of
the full configuration.

To use the new test simply copy basic.dfa to 'pngusr.dfa' in the root of
the source directory and build.

Signed-off-by: John Bowler <jbowler@acm.org>

tests/pngtest-all

@@ -1,17 +1,24 @@
 #!/bin/sh
 
-st=0    # exit status (set to 1 if a test fails)
+st=0        # exit status (set to 1 if a test fails)
+skipped=
 fail="**FAIL**"
+skip="**SKIP**"
 success=" SUCCESS"
 TEST(){
    # Try to make the log file easier to read:
-   test_status="$success"
-   echo "=============== PNGTEST $* ===================="
-   ./pngtest "$@" || {
-      st=$?
-      test_status="$fail"
-   }
+   echo "=============== pngtest $* ===================="
+   ./pngtest "$@"
+   status=$?
+   case "$status" in
+       0)  test_status="$success";;
+       77) test_status="$skip"
+           skipped=1;;
+       *)  test_status="$fail"
+           st="$status";; 
+   esac
    echo "===============$test_status $* ===================="
+   return "$status"
 }
 
 # The "standard" test
@@ -48,15 +55,17 @@ check_stdout(){
    # variable 'line' below.  The pattern matching ignores this because of the
    # '*' at the end of the pattern match.
    found=
+   skipped=
    while read line
    do
       case "$line" in
          *"$2"*) found=1;;
+         *"TEST SKIPPED"*) skipped=1;;
       esac
       echo "$line" # preserve the original output verbatim
    done
    # output the missing warning on descriptor 3:
-   test -z "$found" && echo "$1: $2" >&3
+   test -z "$found" -a -z "$skipped" && echo "$1: $2" >&3
 }
 # NOTE: traditionally the Bourne shell executed the last element in a pipe
 # sequence in the original shell so it could set variables in the original
@@ -74,7 +83,10 @@ exec 4>&1 # original stdout - the log file
      # The exit code is ignored here, the test is that the particular errors
      # (warnings) are produced.  The original output still ends up in the log
      # file.
-     TEST "$file" |
+     {
+        TEST "$file"
+        test "$?" -eq 77 && echo "TEST SKIPPED"
+     } |
         check_stdout "$file" 'IDAT: Read palette index exceeding num_palette' |
         check_stdout "$file" 'Wrote palette index exceeding num_palette' >&4
    done
@@ -90,4 +102,6 @@ exec 4>&1 # original stdout - the log file
    exit $st
 } || st=$?
 
-exit $st
+test "$st" -gt 0 && exit "$st"
+test -n "$skipped" && exit 77
+exit 0