8371914: PNG defines in CFLAGS can cause compilation errors with external libpng

[bsdkurt]

When building with external libpng the PNG defines added to CFLAGS may conflict with the installed libpng header pnglibconf.h resulting in compilation errors. For example on OpenBSD/aarch64:

/usr/local/include/pnglibconf.h:207:9: error: 'PNG_ARM_NEON_OPT' macro redefined [-Werror,-Wmacro-redefined]

This moves all -DPNG_* into the case where internal libpng is used. This has only been tested with the tier1 github actions so needs to be checked on Linux/ppc and AIX.

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• JDK-8371914: PNG defines in CFLAGS can cause compilation errors with external libpng (Bug - P4)

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