Machine files: pkgconfig field deprecated and replaced by pkg-config

Meson used to allow both pkgconfig and pkg-config entries in machine files, the former was used for dependency() lookup and the latter was used as return value for find_program('pkg-config').

This inconsistency is now fixed by deprecating pkgconfig in favor of pkg-config which matches the name of the binary. For backward compatibility it is still allowed to define both with the same value, in that case no deprecation warning is printed.