In CMake, copy the stdlib over if they get changed

My first stab at writing CMake rules

Makefile

@@ -749,10 +749,10 @@ $(call link,_release,$(OPT_OBJS),$(LDFLAGS_RELEASE),$(LLVM_RELEASE_DEPS))
 else
 .PHONY: pyston_dbg pyston_release
 pyston_dbg:
-	$(NINJA) -C $(HOME)/pyston-build-dbg pyston $(NINJAFLAGS)
+	$(NINJA) -C $(HOME)/pyston-build-dbg pyston copy_stdlib $(NINJAFLAGS)
 	ln -sf $(HOME)/pyston-build-dbg/pyston pyston_dbg
 pyston_release:
-	$(NINJA) -C $(HOME)/pyston-build-release pyston $(NINJAFLAGS)
+	$(NINJA) -C $(HOME)/pyston-build-release pyston copy_stdlib $(NINJAFLAGS)
 	ln -sf $(HOME)/pyston-build-release/pyston pyston_release
 endif
 

lib_python/CMakeLists.txt

+# Copy any changed stdlib files to the destination:
+file(GLOB_RECURSE STDLIB_SRCS 2.7/ "*.py")
+SET(STDLIB_TARGETS "")
+foreach(STDLIB_FILE ${STDLIB_SRCS})
+    file(RELATIVE_PATH FN_REL ${CMAKE_SOURCE_DIR} ${STDLIB_FILE})
+    set(TARGET ${CMAKE_BINARY_DIR}/${FN_REL})
+
+    add_custom_command(OUTPUT ${TARGET} COMMAND
+                       ${CMAKE_COMMAND} -E copy_if_different ${STDLIB_FILE} ${TARGET}
+                       DEPENDS ${STDLIB_FILE}
+                       COMMENT "Copying ${FN_REL}"
+                       )
+    set(STDLIB_TARGETS ${STDLIB_TARGETS} ${TARGET})
+endforeach(STDLIB_FILE)
+add_custom_target(copy_stdlib ALL DEPENDS ${STDLIB_TARGETS})
+
 # compile specified files in lib_python/2.7_Modules
 file(GLOB_RECURSE STDMODULE_SRCS 2.7_Modules errnomodule.c shamodule.c sha256module.c sha512module.c _math.c mathmodule.c md5.c md5module.c _randommodule.c _sre.c operator.c binascii.c pwdmodule.c posixmodule.c)