build: use PKG_CHECK_MODULES
All the separate pkg-config calls were done after reading: http://tirania.org/blog/archive/2012/Oct-20.html ("Drowning Good Ideas with Bloat. The tale of pkg.m4." by Miguel de Icaza) But all the GNOME modules I contribute to use PKG_CHECK_MODULES or AX_PKG_CHECK_MODULES for a library. With those macros, it shows all the errors at once (if several libs are missing or too old) instead of aborting at the first error.