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

configure.ac

@@ -228,7 +228,7 @@ LT_OUTPUT
 AC_SYS_LARGEFILE
 
 AM_PROG_AS
-AC_PATH_PROG(NM, nm, nm)
+AC_PATH_TOOL(NM, nm, nm)
 
 AC_MSG_CHECKING([for some Win32 platform])
 case "$host" in
@@ -1444,7 +1444,7 @@ AC_ARG_ENABLE(cups,
 if test "x$enable_cups" = "xno"; then
   AM_CONDITIONAL(HAVE_CUPS, false)
 else
-  AC_PATH_PROG(CUPS_CONFIG, cups-config, no)
+  AC_PATH_TOOL(CUPS_CONFIG, cups-config, no)
   if test "x$CUPS_CONFIG" = "xno"; then
     if test "x$enable_cups" = "xauto"; then
       AM_CONDITIONAL(HAVE_CUPS, false)