Add scripts/makefile.c89 and refactor scripts/makefile.emcc,
scripts/makefile.clang and scripts/makefile.gcc
Refactor variable definitions inside scripts/makefile.clang,
scripts/makefile.gcc and scripts/makefile.emcc, and start using
the option `-pedantic-errors` unconditionally. This option was
first implemented in GCC version 3.1, and it was available in
Clang and in other Clang-based compilers (e.g. Emscripten) from
the beginning.
Add scripts/makefile.c89, derived from the above makefiles, but
with `-pedantic-errors -std=c89`. We aren't enabling the C89 level
by default, to avoid any incompatibility, whether intentional or
accidental, with the compiler's default language level. However,
we are still continuing to support C89 in the 'libpng16' branch,
and this special makefile can be used for testing purposes.
Split AR_RC into AR and ARFLAGS. The variables AR and ARFLAGS are
de-facto standards (like CC and CFLAGS, LD and LDFLAGS, etc.) that
may be overridden when running make. Moreover, configuring CC, LD,
AR, etc., to point to a cross-platform compiler, linker, librarian,
etc., is a de-facto standard practice as well.
Also remove the MKDIR_P variable definitions from all makefiles.
They've been leftovers from the removal of the "install*" targets.
