CMake: don't overwrite the user's CFLAGS

If either CFLAGS is defined or the user passes -DCMAKE_C_FLAGS to
cmake, the variable already contains flags. Don't overwrite them, but
append them to our settings.

Signed-off-by: Carlos Martín Nieto <carlos@cmartin.tk>

CMakeLists.txt

@@ -57,7 +57,7 @@ ENDIF ()
 
 # Platform specific compilation flags
 IF (MSVC)
-	SET(CMAKE_C_FLAGS "/W4 /nologo /Zi")
+	SET(CMAKE_C_FLAGS "/W4 /nologo /Zi ${CMAKE_C_FLAGS}")
 	IF (STDCALL)
 	  SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} /Gz")
 	ENDIF ()
@@ -65,7 +65,7 @@ IF (MSVC)
 	SET(CMAKE_C_FLAGS_DEBUG "/Od /DEBUG /MTd")
 	SET(CMAKE_C_FLAGS_RELEASE "/MT /O2")
 ELSE ()
-	SET(CMAKE_C_FLAGS "-O2 -g -Wall -Wextra -Wstrict-aliasing=2 -Wstrict-prototypes -Wmissing-prototypes")
+	SET(CMAKE_C_FLAGS "-O2 -g -Wall -Wextra -Wstrict-aliasing=2 -Wstrict-prototypes -Wmissing-prototypes ${CMAKE_C_FLAGS}")
 	IF (NOT MINGW) # MinGW always does PIC and complains if we tell it to
 		SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fPIC")
 	ENDIF ()