Some minor (albeit incomplete) MingW fixes.

configure.in

@@ -399,8 +399,13 @@ fi
 AC_MSG_RESULT([$this_is_cygwin])
 
 AC_MSG_CHECKING([if this is mingw])
+if test x$target_os = xmingw32; then
+  this_is_mingw=yes
+fi
 if test x$target_os = xmingw32msvc; then
   this_is_mingw=yes
+fi
+if test x$this_is_mingw = xyes; then
   PHYSFSCFLAGS="$PHYSFSCFLAGS -DWIN32"
   enable_cdrom=yes
   enable_pthreads=no

platform/win32.c

@@ -25,7 +25,7 @@
 
 #if (defined _MSC_VER)
     #define alloca(x) _alloca(x)
-#elif (defined MINGW)  /* scary...hopefully this is okay. */
+#elif (defined __MINGW32__)  /* scary...hopefully this is okay. */
     #define alloca(x) __builtin_alloca(x) 
 #endif