libtool: Don't hardcode grep paths

If sharing sstate between different distros, some of which have /bin/grep
and some /usr/bin/grep, this can break. Simply don't hardcode the path.

(From OE-Core rev: f6e7a3b1ac75c274493666b9a31cca83b7d2138b)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-devtools/libtool/libtool-2.4.6.inc

@@ -35,6 +35,8 @@ do_compile_prepend () {
 inherit autotools texinfo
 EXTRA_AUTORECONF = "--exclude=libtoolize"
 
+CACHED_CONFIGUREVARS += "ac_cv_path_GREP=grep"
+
 DEPENDS = "libtool-native"
 
 PACKAGES =+ "libltdl"