build: Fix compiler optimisation flags

Ensure the flags apply to the C compiler, not C++, as there is no C++ in
this core.
With this, the Miyoo platform can reach 60fps.

Author: jSTE0

Makefile

@@ -593,9 +593,9 @@ ifeq (,$(findstring msvc,$(platform)))
 endif
 
 ifeq ($(DEBUG), 1)
-	CXXFLAGS += -O0 -g
+	CFLAGS += -O0 -g
 else
-	CXXFLAGS += -O2 -DNDEBUG
+	CFLAGS += -O2 -DNDEBUG
 endif
 
 ifneq (,$(findstring msvc,$(platform)))