Add CMake build script

[ccawley2011]

This is based on the CMake scripts for libogg and libvorbis, and should hopefully be more general purpose than the previous PR.

[sezero]

This is going at a much better direction than before.

A few missing things:

• detection of C99 VLA support:
  https://github.com/libsdl-org/tremor/blob/sezero/configure.ac#L100
  (I guess opus project's cmake may have an equivalent)

• option to disable alloca (and use vla - see above):
  https://github.com/libsdl-org/tremor/blob/sezero/configure.ac#L100

• version script support:
  https://github.com/libsdl-org/tremor/blob/sezero/configure.ac#L160
  Which requires GNU-ld compatible linker, a cmake check for it
  is present in our libxmp:
  https://github.com/libxmp/libxmp/blob/master/cmake/libxmp-checks.cmake#L20-L30

[sezero]

I guess this is obsolete after today's changes by @madebr?

[sezero]

I can, however, use this as a base in my original fork after addressing the points I mentioned above.

[madebr]

This is not obsolete.
I added the absolute minimum to get it building as a subproject with SDL_mixer.
These additions make it possible to build tremor as a standalone library.
(I did not see this pr before adding my cmake script)

[sezero]

OK then.

[sezero]

Cameron: This conflicts with the @madebr's existing cmake script, and is still missing the items I mentioned above.