Add CMakeLists.txt.

Initial start to this stolen from:

    https://github.com/johnwbyrd/libsndfile-cmake
CMakeLists.txt : First pass at making it work for Linux.

Cmake is great for really simple stuff but is at least as difficult
as autotools for more complicated stuff.
autogen.sh : Generate CMake/config.h.in from src/config.h.in.
CMake : Detect and set COMPILER_IS_GCC.
CMake : Use check_type_size as needed.
CMake : Split functions out into a separate file.
CMake : Add all the library C files and start on CFLAGS.
CMakeLists.txt : Build a versioned shared library.
CMake : Detect functions and libraries.