Only test for .CPP on Microsoft compilers which will handle .CPP as c++.

Tests/CxxOnly/CMakeLists.txt

@@ -2,7 +2,10 @@
 project (CxxOnly CXX)
 
 set(CMAKE_DEBUG_POSTFIX "_test_debug_postfix")
-add_library(testcxx1 STATIC libcxx1.cxx test.CPP)
+if(MSVC)
+  set(EXTRA_SRCS test.CPP)
+endif()
+add_library(testcxx1 STATIC libcxx1.cxx ${EXTRA_SRCS})
 add_library(testcxx2 SHARED libcxx2.cxx)
 add_executable (CxxOnly cxxonly.cxx)
 target_link_libraries(CxxOnly testcxx1 testcxx2)

Tests/CxxOnly/cxxonly.cxx

@@ -1,12 +1,16 @@
 #include "libcxx1.h"
 #include "libcxx2.h"
+#ifdef _MSC_VER
 extern int testCPP;
+#endif
 
 #include <stdio.h>
 
 int main ()
 {
+#ifdef _MSC_VER
   testCPP = 1;
+#endif
   if ( LibCxx1Class::Method() != 2.0 )
     {
     printf("Problem with libcxx1\n");