cmake: Add to CMAKE_MODULE_PATH instead of clobbering it.
cmake: Do not build Unix-only files under VxWorks.
Do not build demangle.cc under VxWorks.

The VxWorks 6.9 GNU toolchain provides <cxxabi.h>, but the libraries
do not provide abi::__cxa_demangle() [or __cxa_atexit() or
__cxa_finalize(); the rest of the functions declared in cxxabi.h
appear to be available].  So a configuration check for <cxxabi.h> (as
suggested in the demangle.cc source) will not work.