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

Remove hacks to force language from CMake files.

Closes KhronosGroup/OpenCL-CTS#25

test_conformance/commonfns/CMakeLists.txt

@@ -1,25 +1,25 @@
 set(MODULE_NAME COMMONFNS)
 
 set(${MODULE_NAME}_SOURCES
-    main.c
-    test_clamp.c
-    test_degrees.c
-    test_max.c
-    test_maxf.c
-    test_min.c
-    test_minf.c
-    test_mix.c
-    test_radians.c
-    test_step.c
-    test_stepf.c
-    test_smoothstep.c
-    test_smoothstepf.c
-    test_sign.c
-    test_fmax.c
-    test_fmin.c
-    test_fmaxf.c
-    test_fminf.c
-    test_binary_fn.c
+    main.cpp
+    test_clamp.cpp
+    test_degrees.cpp
+    test_max.cpp
+    test_maxf.cpp
+    test_min.cpp
+    test_minf.cpp
+    test_mix.cpp
+    test_radians.cpp
+    test_step.cpp
+    test_stepf.cpp
+    test_smoothstep.cpp
+    test_smoothstepf.cpp
+    test_sign.cpp
+    test_fmax.cpp
+    test_fmin.cpp
+    test_fmaxf.cpp
+    test_fminf.cpp
+    test_binary_fn.cpp
 )
 
 include(../CMakeCommon.txt)