Fix GH-14443: Cross-compiling with external GD library

[petk]

This enables cross-compiling simplifications when using external GD library as requested in GH-14443 using cache variables:

• php_cv_lib_gd_gdImageCreateFromPng
• php_cv_lib_gd_gdImageCreateFromAvif
• php_cv_lib_gd_gdImageCreateFromWebp
• php_cv_lib_gd_gdImageCreateFromJpeg
• php_cv_lib_gd_gdImageCreateFromXpm
• php_cv_lib_gd_gdImageCreateFromBmp
• php_cv_lib_gd_gdImageCreateFromTga

For example:

./configure --host=... --build=... --enable-gd --with-external-gd \
    php_cv_lib_gd_gdImageCreateFromPng=yes

Note that this has already been implemented in master branch (PHP-8.4-dev) and this one is targeting PHP-8.2.

[petk]

I'll also adjust these help texts because they are not what they say at the moment. They are actually if the libgd is built with support for certain library...

[petk]

I've just found that libgd uses pkg-config's Requires.private which can be an option to simplify the cross-compilation completely without using these cache variables but the macro will become a bit more complex...

cc @orlitzky I'll append one possible solution here.

[petk]

I'll just close this as it's quite a while since I was checking this and according to current understanding cross-compilation can't be simplified without running a test program. Current master (PHP-8.5) branch and even PHP-8.4 have this mostly resolved and cache variables can be set in edge cases where cross-compiling and wanting to use external libgd.