[INSTALL] Mention pkg-config requirement

Mention that building the configure script requires at least version 0.16
of pkg-config.

See the old bug:
  Bug 4702 PKG_PROG_PKG_CONFIG: command not found
  (https://bugs.freedesktop.org/show_bug.cgi?id=4702)

INSTALL

@@ -160,6 +160,7 @@ order to compile cairo. Specifically, you need the following utilities:
 	autoheader
 	aclocal
 	libtoolize
+	pkg-config [at least version 0.16]
 	gtk-doc (recommended)
 
 Hopefully your platform of choice has packages readily available so