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

Remove hacks to force language from CMake files.

Closes KhronosGroup/OpenCL-CTS#25

test_conformance/printf/CMakeLists.txt

@@ -3,8 +3,8 @@ set(MODULE_NAME PRINTF)
 set(CMAKE_CXX_STANDARD 11)
 
 set(${MODULE_NAME}_SOURCES
-         test_printf.c
-         util_printf.c
+         test_printf.cpp
+         util_printf.cpp
          )
 
 include(../CMakeCommon.txt)