CMakeLists.txt points at missing file

[JayFoxRox]

18e3b16 introduced a bug which breaks CMake (which was added later in 4246158); this prevents running tests on non-Xbox platforms.

$ mkdir build
$ cd build
$ cmake ..
++ CMAKE_BUILD_TYPE not set. Defaulting to 'Debug'.
-- Configuring done
CMake Error at CMakeLists.txt:309 (add_library):
  Cannot find source file:

    functions/_PDCLIB/assert.c

  Tried extensions .c .C .c++ .cc .cpp .cxx .cu .m .M .mm .h .hh .h++ .hm
  .hpp .hxx .in .txx

[...]

I'm not sure how to resolve this most gracefully.

Generally it feels like 18e3b16 shouldn't require changes to existing pdclib code. We have extension hooks now and there has always been the optimization overlay (which works as long as we don't use wildcards in our Makefiles).