Added build option M32 for x86 builds on x64 platforms.

By using the new option one may compile a 32-bit version of the game on
64-bit platforms by specifying:
$ cmake -DM32=1

Of course this would require the gcc-multilib package and also 32-bit
versions of libSDL and FMod Ex shared objects.
CMake parameter -DNO_SOUND=1 changed to -DSOUND=NO
Fixes in fmodex linking and executable RPATH.

Change: FmodEx shared object libs now first searched for in
./libs/fmod/api. Only if not found there the /usr/lib, etc. default
folders will be searched. This is needed because of 32-bit build on
64-bit platforms where we need to locate the proper libfmodex.so.

Change: CMake has an option CMAKE_INSTALL_RPATH_USE_LINK_PATH that is
helps locating required libraries independent from LD_LIBRARY_PATH and
the install location. Right now when building install packages (cpack)
the executable will have its RPATH stripped. ($ readelf -d savagewheels)