FreeBSD: recommend to use gmake instead of make

Command `make` on FreeBSD doesn't like MXE's Makefile:

make: "/root/mxe2/Makefile" line 10: Cannot open /ext/gmsl
make: "/root/mxe2/Makefile" line 59: Need an operator
make: "/root/mxe2/Makefile" line 62: Need an operator
make: "/root/mxe2/Makefile" line 126: Need an operator
make: "/root/mxe2/Makefile" line 130: Need an operator
...

$(MAKEFILE_LIST) is empty for `make` on FreeBSD, that is why
$(EXT_DIR) is /ext

Nevertheless MXE's Makefile seems to use too many features
not supported by `make` on FreeBSD. Tool `gmake` is installed.

index.html

@@ -670,6 +670,9 @@ USE_OSGPLUGIN(<plugin2>)
     libtool openssl p5-XML-Parser p7zip patch perl5 \
     pkgconf python ruby scons unzip wget</pre>
 
+    <p>
+    Use gmake instead of make.
+    </p>
     <p>
     Ensure that /usr/local/bin precedes /usr/bin in your $PATH:
     </p>