Support for pkg-config .pc files

Hello,

There is another pull request in preparation that I'd like to squeeze in
the PHP-8.4 branch if it will possible to wrap it up until the feature
freeze milestone:
https://github.com/php/php-src/pull/13755

In short, pkg-config is *nix command line utility to query installed
libraries on the system:
https://en.wikipedia.org/wiki/Pkg-config
https://people.freedesktop.org/~dbn/pkg-config-guide.html

And pkgconf is a compatible maintained continuation/fork that many
distributions might also use.

Each library creates a so-called .pc file, for example, php.pc which is
installed to the pkgconfig subdirectory. For example,
/usr/lib/x86_64-linux-gnu/pkgconfig/php.pc

.pc file is created from a template .pc.in and gets some placeholders
replaced during the configuration phase of building to get the library
name, include flags etc.

PHP already extensively relies on pkg-config's interface for finding
certain dependencies during the build (libxml2, SQLite, OpenSSL, curl...)
but so far never implemented its own .pc file(s).

PHP could have two such .pc files. One for a global PHP installation as a
whole that PHP extensions can use (php.pc) and one for the embed SAPI to
embed PHP into an application (php-embed.pc)

Its usage is very similar to php-config script, i.e. getting build details.

See PR for details, I'll update it on the way what can be done.

Let me know if that's fine or is there some concern.