m4macros: Use PKG_PROG_PKG_CONFIG
AM_PATH_GTK_3_0 uses AC_PATH_PROG for finding pkg-config. Unfortunately, that will find the build architecture pkg-config which in turn will miss the host architecture gtk+3.0.
Based on a patch from Helmut Grohne via Debian bug 894069. Fixes: #133 (closed).
This is a less change-averse version than !80 (merged).