Update `string(TOLOWER ...)` to take string versions of CMake variables (#561)

On some generators (MSVC) there is the issue that this line produces cause by the variable being expanded and not being surrounded by quotes.

CMakeLists.txt

@@ -199,8 +199,8 @@ if(CMAKE_SIZEOF_VOID_P EQUAL 8)
 	set(BITS 64)
 endif(CMAKE_SIZEOF_VOID_P EQUAL 8)
 
-string(TOLOWER ${CMAKE_SYSTEM_NAME} SYSTEM_NAME)
-string(TOLOWER ${CMAKE_BUILD_TYPE} BUILD_TYPE)
+string(TOLOWER "${CMAKE_SYSTEM_NAME}" SYSTEM_NAME)
+string(TOLOWER "${CMAKE_BUILD_TYPE}" BUILD_TYPE)
 
 if(ANDROID)
 	# Added the android abi after system name