Rename test .c sources to .cpp where necessary (#604)

Remove hacks to force language from CMake files.

Closes KhronosGroup/OpenCL-CTS#25

test_conformance/half/CMakeLists.txt

@@ -1,10 +1,10 @@
 set(MODULE_NAME HALF)
 
 set(${MODULE_NAME}_SOURCES
-        cl_utils.c
-        Test_vLoadHalf.c
-        Test_roundTrip.c
-        Test_vStoreHalf.c main.c
+        cl_utils.cpp
+        Test_vLoadHalf.cpp
+        Test_roundTrip.cpp
+        Test_vStoreHalf.cpp main.cpp
 )
 
 include(../CMakeCommon.txt)