Added GNU Assembler toolset support and fixed build problem on Debian 6.0.

Hey, I needed to be able to use the GNU Assembler from within tundra for one of my projects, so I just copied your existing YASM toolset implementation in yasm.lua, and modified it for the GNU Assembler, creating scripts/tundra/tools/gas.lua. Thought it would be a good idea to make this available.

I also ran into a problem trying to get tundra to build on distributions with older versions of glibc/eglibc, which don't make available the POSIX.1-2008 interfaces by default. So the build would fail out when it tried to use a few of the more recently standardized pthread features like PTHREAD_MUTEX_RECURSIVE. The work around is to make sure _GNU_SOURCE preprocessor definition is defined during compilation, so I simply updated both CMakeLists.txt and tundra.lua to do just that. It doesn't cause any forward compatibility problems with newer glibc/eglibc versions.

This allowed me to be able to build tundra on a vanilla Debian 6.0 installation without trouble.