Look for a cross-compile libtool first in configure.

Permit cross-compilation for Darwin.

configure

@@ -266,7 +266,9 @@ if test "$gcc" -eq 1 && ($cc -c $test.c) >> configure.log 2>&1; then
         SHAREDLIBV=libz.$VER$shared_ext
         SHAREDLIBM=libz.$VER1$shared_ext
         LDSHARED=${LDSHARED-"$cc -dynamiclib -install_name $libdir/$SHAREDLIBM -compatibility_version $VER1 -current_version $VER3"}
-        if libtool -V 2>&1 | grep Apple > /dev/null; then
+        if "${CROSS_PREFIX}libtool" -V 2>&1 | grep Apple > /dev/null; then
+            AR="${CROSS_PREFIX}libtool"
+        elif libtool -V 2>&1 | grep Apple > /dev/null; then
             AR="libtool"
         else
             AR="/usr/bin/libtool"