meson, configure, gtk: fix GDK pkg-config for macOS.

Since commit 7e732caa, gdkquartz.h is included directly from gdkquartz-cocoa-access.h, yet it is in the include/gtk+-3.0/gdk subdirectory (which is currently not in the dir list to search for headers). It is made so on purpose so that the headers is also found in the source tree. Yet it also definitely needs to be found in the install prefix tree, which is the job of the pkg-config file.

See this comment and following messages: gimp!535 (comment 1343169)