Backport from devbranch: mingw support.

CHANGELOG

@@ -4,6 +4,7 @@
 
 -- stuff in the stable-1.0 branch, backported from 2.0.0 dev branch, etc ---
 
+06122005 - Added support for mingw to Unix build process (thanks, Matze!).
 03162005 - Readded Spanish translation. Added Brazillian Portguese translation
            and fixed unlocalized string in wad.c (thanks Danny!). Some MacOS
            Classic fixes and MPW support (thanks, Chris!). Changed CWProjects

CREDITS

@@ -73,6 +73,9 @@ Brazillian Portuguese translation:
 Spanish translation:
     Pedro J. Pérez
 
+Mingw support:
+    Matze Braun
+
 Other stuff:
     Your name here! Patches go to icculus@clutteredmind.org ...
 

configure.in

@@ -380,9 +380,20 @@ if test x$target_os = xcygwin; then
 else
   this_is_cygwin=no
 fi
-
 AC_MSG_RESULT([$this_is_cygwin])
 
+AC_MSG_CHECKING([if this is mingw])
+if test x$target_os = xmingw32msvc; then
+  this_is_mingw=yes
+  PHYSFSCFLAGS="$PHYSFSCFLAGS -DWIN32"
+  enable_cdrom=yes
+  enable_pthreads=no
+  have_non_posix_threads=yes
+else
+  this_is_mingw=no
+fi
+AC_MSG_RESULT([$this_is_mingw])
+
 this_is_macosx=no
 if test x$we_have_sed = xyes; then
   AC_MSG_CHECKING([if this is MacOS X])