AM_PATH_GTK_3_0: Use AC_PATH_TOOL to find 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. What must be used here is the host architecture pkg-config and that is found with AC_PATH_TOOL.
Patch from Helmut Grohne via Debian bug 894069. Fixes: #133 (closed).
!81 (merged) is a less change-averse version of this.
Edited by Simon McVittie