Add cmake build infrastructure.

Add cmake build infrastructure.
Not default nor even distributed in the .tar.gz, the cmake build allows for
loadable yasm plugins by building libyasm as a shared library.
Example plugins are in the plugins/ directory, and may be loaded into a
cmake-built yasm using the -N command line option (non-cmake builds will
not have this option).

Tested only on Linux so far, but should be relatively painless to port to
Windows thanks to the use of cmake rather than libtool to create shared
libraries.

The only modification to the main source tree is some conditional-compiled
additions to yasm.c.

svn path=/trunk/yasm/; revision=2098