AM_PATH_GTK_3_0: Use AC_PATH_TOOL to find pkg-config

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.

Bug-Debian: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=894069
Reviewed-by: Simon McVittie <smcv@debian.org>
Bug: https://gitlab.gnome.org/GNOME/gtk/issues/133
Applied-upstream: 3.22.30, commit:3a7fbb3b30f6ed8f65f9f5862c2fd30035a1a1ed

Gbp-Pq: Name 0014-AM_PATH_GTK_3_0-Use-AC_PATH_TOOL-to-find-pkg-config.patch