Support more C++ file extensions for MSVC in the compile script.

* lib/compile (func_cl_wrapper): MSVC only recognizes the .cpp
file extension as C++, unless it's given a hint. So hint about
.cc, .CC, .cxx, .CXX, c++ and C++. Also do path conversion on
.c, .cpp, .CPP, .lib, .LIB and .Lib files.
* lib/compile3.test: Test the C++ hinting.

Signed-off-by: Peter Rosin <peda@lysator.liu.se>