EXTENSION_MAPPING for cpp like language

[ShuguangSun]

Is your feature request related to a problem? Please describe.
I'm writing SAS code which has simliar block comments like c/cpp "/* ... */". I can use "EXTENSION_MAPPING = sas=C++" to tell doxygen to treat the file description as C++ files. However, sas will be recognized as SAS language as there is a sas lsp.

Describe the solution you'd like
A configure value about extension mapping which tell the extension treat sas file as C++.