Default to glibtoolize instead of libtoolize if it exists,

and also make it customizable through the environment variable
$LIBTOOLIZE.

Fixes autogen.sh issues on OS X, which ships its own
(incompatible) libtoolize.

R=jeff

autogen.sh

@@ -2,6 +2,11 @@
 rm -rf autom4te.cache
 aclocal -I m4
 autoheader
-libtoolize --copy
+if glibtoolize --version >/dev/null 2>/dev/null; then
+  LIBTOOLIZE=${LIBTOOLIZE:-glibtoolize}
+else
+  LIBTOOLIZE=${LIBTOOLIZE:-libtoolize}
+fi
+$LIBTOOLIZE --copy
 automake --add-missing --copy
 autoconf