RFR: 8371914: PNG defines in CFLAGS can cause compilation errors with external libpng [v2]

[Erik Joelsson]

On Wed, 3 Dec 2025 15:50:00 GMT, Kurt Miller <kurt at openjdk.org> wrote:

>> 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.
>
> Kurt Miller has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Clarify comment per code review

Marked as reviewed by erikj (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/28324#pullrequestreview-3536379873