Fix typo in pkg.m4

[eltociear]

occurence -> occurrence

[nikic]

This is a bundled file that we don't maintain -- so any fixes should go upstream first. Unfortunately I wasn't able to quickly determine where that upstream actually is. It doesn't seem to be https://github.com/pkgconf/pkgconf/blob/master/pkg.m4 which uses an older version than we do (serial 11 instead of serial 12).

@petk Do you know where this file originally comes from? It's not listed in https://github.com/php/php-src/blob/master/CONTRIBUTING.md#php-source-code-directory-structure.

[petk]

Hello, the Git repository is here https://gitlab.freedesktop.org/pkg-config/pkg-config
and this typo has already been fixed in the upstream file:
https://gitlab.freedesktop.org/pkg-config/pkg-config/-/commit/eb866ade77d933c3237a5c9e2114ca65c3baa537

So, once the release will be done there (currently it is still at the 0.29.2) we'll bump it also here.

Otherwise, thank you @eltociear. Instead of this typo fix, you could maybe add the link to pkg.m4 in the docs file mentioned by @nikic