Update Bootstrap.mak
Fix build failed in mingw x86 with v5.0.0-beta1 version. Here is the output:
Makefile:34: *** "invalid configuration release_x86".  Stop.
mingw32-make: *** [Bootstrap.mak:91: mingw] Error 2
@@ -88,7 +88,7 @@ mingw: mingw-clean
$(CC) -o build/bootstrap/premake_bootstrap -DPREMAKE_NO_BUILTIN_SCRIPTS -I"$(LUA_DIR)" -I"$(LUASHIM_DIR)" $(SRC) -lole32 -lversion
./build/bootstrap/premake_bootstrap embed
./build/bootstrap/premake_bootstrap --arch=$(PLATFORM) --os=windows --to=build/bootstrap --cc=mingw gmake2
- $(MAKE) -C build/bootstrap config=$(CONFIG)_$(PLATFORM)
+ $(MAKE) -C build/bootstrap config=$(CONFIG)_$(PLATFORM:x86=win32)
macosx: osx
