[PATCH 0/1] MR9313: configure: Only run checks in WINE_(MINGW_)PACKAGE_FLAGS when CFLAGS or LIBS is non-empty.

[Brendan Shanks (＠bshanks)]

Don't run the checks unless `_CFLAGS` or `_LIBS` is non-empty (either because they were specified manually or `pkg-config` found the package).

This fixes an issue seen on macOS: PKG_CONFIG_LIBDIR and PKG_CONFIG_PATH were set empty to prevent pkg-config from using any system-installed packages, but FFmpeg was installed in /usr/local and AC_CHECK_HEADERS/LIBS succeeded despite FFMPEG_CFLAGS/LIBS being empty. configure printed "FFmpeg development files not found.", but HAVE_FFMPEG was still set. The winedmo files compiled, but failed to link because FFMPEG_LIBS was empty.
This should also speed up configure slightly, by skipping header checks for not-installed packages.

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https://gitlab.winehq.org/wine/wine/-/merge_requests/9313