CMakeLists: Fix cmake warnings

CMakeLists: Fix cmake warnings

Since cmake version 3.1, variable reference policy is changed, and
'PREFIX' defined for refer '/usr/' directory cause warnings during
dealing with internal scripts like below:

	#if defined(__VISUALDSPVERSION__)
	    /* __VISUALDSPVERSION__ = 0xVVRRPP00 */
	# define @PREFIX@COMPILER_VERSION_MAJOR @MACRO_HEX@(__VISUALDSPVERSION__>>24)
	# define @PREFIX@COMPILER_VERSION_MINOR @MACRO_HEX@(__VISUALDSPVERSION__>>16 & 0xFF)
	# define @PREFIX@COMPILER_VERSION_PATCH @MACRO_HEX@(__VISUALDSPVERSION__>>8  & 0xFF)
	#endif'

To fix it, this changes to not use PREFIX as pre-defined variable no
longer.

Change-Id: Ie62509318b84075b8a03be0e806696b2837ecbd9
Signed-off-by: Dongwoo Lee <dwoo08.lee@samsung.com>