[mingw] switched to autotools for Makefile generation

* allows better integration with Clang
* use ./autogen.sh to generate compilation scripts
* old Makefile renamed to mf so that make -f mf can still be invoked
* also fixed one Clang warning

autogen.sh

@@ -0,0 +1,11 @@
+#!/bin/sh
+
+# use glibtoolize if it is available
+(glibtoolize --version) < /dev/null > /dev/null 2>&1 && LIBTOOLIZE=glibtoolize || LIBTOOLIZE=libtoolize
+
+$LIBTOOLIZE --copy --force || exit 1
+aclocal || exit 1
+autoheader || exit 1
+autoconf || exit 1
+automake -a -c || exit 1
+./configure --disable-debug $*