Minor fixes in Makefiles:

- Project Makefile: Fallback $OS in linux to $(UNAME_S) which uses `uname -s`  in linux, since $(OS) is specific to windows.
- Luac Cross: add .exe when compiling for windows.

Author: caiohamamura

app/lua/luac_cross/Makefile

@@ -88,6 +88,10 @@ else
 IMAGE := ../../../luac.cross.int
 endif
 
+ifeq ($(OS),Windows_NT)
+  IMAGE := $(IMAGE).exe
+endif
+
 .PHONY: test clean all
 
 all: $(DEPS) $(IMAGE)

app/lua53/host/Makefile

@@ -90,6 +90,10 @@ else
 IMAGE := ../../../luac.cross.int
 endif
 
+ifeq ($(OS),Windows_NT)
+  IMAGE := $(IMAGE).exe
+endif
+
 .PHONY: test clean all
 
 all: $(DEPS) $(IMAGE)