Use .cpp as the extension for temporary C++ files.

Using a .c extension for a C++ file raises the following warning, which
breaks our header file tests if the compiler is using -Werror as well:

c++: warning: treating 'c' input as 'c++' when in C++ mode, this
behavior is deprecated

Obtained from: atf (git 3104010c2849330440cc0ce108ff341913433339)
MFC after: 3 days