Autotools: Add pkg-config for GMP library

[petk]

This can also wait for the next PHP version or perhaps for PHP 8.4 beta otherwise it seems to be working fine.

GMP has pkg-config integration since 2019-08-22 (version ~6.2.0).

This optionally finds the GMP library using pkg-config or falls back to find library on the system or with the provided configure option argument (--with-gmp=DIR).

When using DIR argument, the pkg-config check is silently skipped.

When not using DIR argument, the GMP_CFLAGS and GMP_LIBS can be also used to find the GMP library:

 ./configure --with-gmp \
     GMP_CFLAGS=-I/path/to/gmp/include \
     GMP_LIBS="-L/path/to/gmp -lgmp" \

[devnexen]

MSTM

[NattyNarwhal]

Could omit the =dir handling and just rely on the PKG_CHECK_MODULES override behaviour for i.e. GMP_CFLAGS?

[Girgias]

Makes sense to me, and this can land in the beta IMHO

[petk]

Could omit the =dir handling and just rely on the PKG_CHECK_MODULES override behaviour for i.e. GMP_CFLAGS?

Yes, I think removing the DIR argument could be possible and not problematic. In the current implementation (as is in the current master branch, not in this PR) this DIR is not working ok on many *nix installations because gmp is actually installed to a compiler default locations (such as machine includes, library path of /usr/lib/x86_64_linux-gnu etc.) So, if pkg-config doesn't find it it falls back to just checking the default paths (like it does already) and if still not found some sensible error is thrown. I'll recheck and test this a bit to not miss anything.

[petk]

Issue with removing the DIR argument --with-gmp=DIR is that it is used quite a lot probably. Even in PHP CI files:

php-src/.github/actions/configure-macos/action.yml

Line 55 in 1251901

--with-gmp="$BREW_OPT"/gmp \

So, I would suggest to go with using optional pkg-config and then resorting to system paths as a fallback. And optionally check the DIR argument here.

[NattyNarwhal]

I feel we could tell people to set the manual overrides or adjust PKG_CONFIG_PATH, but this is also fine as it stands.

[petk]

I feel we could tell people to set the manual overrides or adjust PKG_CONFIG_PATH, but this is also fine as it stands.

Yes, PHP started adopting this pkg-config at some point very heavily. But passing the library installation directory isn't something bad or old. If the library follows some minimum standards and has GNU-like installation directory structure it is actually simpler to use than adjusting pkg-config variables.

Also, just for example, CMake has PackageName_ROOT variables that are somehow similar to these DIR arguments. They of course would like to see CMake configuration exported by the library itself, so that find_package(PackageName) would work out of the box without additional adjustment by the developer using the PackageName but in practice this turned out that most libraries don't follow this. Or if the library chooses Meson build system, then build system specific wishes can't work. Probably the pkg-config should be the defacto standard in C for finding these packages but it is moving very slowly and isn't something very reliable. It is quite difficult to master these build system specific stuff for some library maintainer. Not to mention that pkg-config isn't available on Windows. :/ Yes, C package management is very problematic :D