We have to help `pkg-config` since it can't recognize that there is no
shared library installed.
Note that meson already does exactly the same.
* builds/unix/configure.raw (REQUIRES_PRIVATE, LIBS_PRIVATE): Rename to...
(PKGCONFIG_REQUIRES_PRIVATE, PKGCONFIG_LIBS_PRIVATE): This. Adjust them
depending on `$enable_shared`.
(PKGCONFIG_REQUIRES, PKGCONFIG_LIBS): New variables.
* builds/unix/freetype2.in, builds/unix/unix-def.in: Use new and updated
variables.
119e404b89