Use AC_PATH_TOOL to find build tools.
Use AC_PATH_TOOL to find build tools.

This improves support for cross-compilation & multilib on Gentoo, where
tools proper for target arch/ABI are prefixed with CHOST.

https://bugzilla.gnome.org/show_bug.cgi?id=731582

Distrotech/gtk,Distrotech/gtk,Distrotech/gtk,johne53/MB3Gtk-2,Distrotech/gtk,johne53/MB3Gtk-2,johne53/MB3Gtk-2,Distrotech/gtk,johne53/MB3Gtk-2,johne53/MB3Gtk-2