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

[Kurt Miller]

> 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

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/28324/files
  - new: https://git.openjdk.org/jdk/pull/28324/files/a7c9813f..50061a1d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28324&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28324&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/28324.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28324/head:pull/28324

PR: https://git.openjdk.org/jdk/pull/28324