On Darwin, only use /usr/bin/libtool if libtool is not Apple.

The original change was to always use /usr/bin/libtool on Darwin,
in order to avoid using a GNU libtool installed by the user in the
path ahead of Apple's libtool.  However someone might install a
more recent Apple libtool ahead of /usr/bin/libtool.  This commit
checks to see if libtool is Apple, and uses /usr/bin/libtool if it
isn't.

configure

@@ -231,7 +231,11 @@ if test "$gcc" -eq 1 && ($cc -c $cflags $test.c) >> configure.log 2>&1; then
              SHAREDLIBV=libz.$VER$shared_ext
              SHAREDLIBM=libz.$VER1$shared_ext
              LDSHARED=${LDSHARED-"$cc -dynamiclib -install_name $libdir/$SHAREDLIBM -compatibility_version $VER1 -current_version $VER3"}
-             AR="/usr/bin/libtool"
+             if test "`libtool -V 2>&1 | grep -c Apple`" != "0"; then
+                 AR="libtool"
+             else
+                 AR="/usr/bin/libtool"
+             fi
              ARFLAGS="-o" ;;
   *)             LDSHARED=${LDSHARED-"$cc -shared"} ;;
   esac