Fix building on MSYS2

Makefile.win32

@@ -3,14 +3,15 @@ AR       = ar
 LAUNCHER =
 RM       = rm -rf
 MKDIR    = mkdir
+ECHO     = echo
 
-LIBS=-I/mingw/include/SDL -D_GNU_SOURCE=1 -Dmain=SDL_main -lopengl32
-CFLAGS= -L/mingw/lib -lmingw32 -lSDLmain -lSDL -mwindows -g
-LDFLAGS=
+LIBS    = -L/mingw/lib -Dmain=SDL_main -lopengl32 -lSDLmain -lSDL 
+CFLAGS  = -I/mingw/include -g -mwindows -D_GNU_SOURCE=1
+LDFLAGS =
 
-RES_GAMELIB=libgame.a
-RES_GAME=game.exe
-BUILDDIR=build-mingw
+RES_GAMELIB = libgame.a
+RES_GAME    = game.exe
+BUILDDIR    = build-$(shell gcc -v 2>&1 | grep "Target:" | cut --delimiter=' ' --fields=2)
 
 include Makefile.common