Added toolset support for the GNU Assembler (GAS).

 This is a simple addition to make available the GNU Assembler as
 a separate toolset, which can be used in tandem with GCC or other
 toolsets. It works the same as the existing YASM toolset, simply
 add it to your Tools configuration (eg. Tools = { "gcc", "gas" }).

 The project's tundra.lua and CMakeLists.txt files were also
 updated to define the _GNU_SOURCE preprocessor definition when
 compiling on Linux/UNIX platforms. This was necessary to get it to
 build on platforms with older versions of glibc in which the
 POSIX.1-2008 interfaces aren't made available by default (for
 example, PTHREAD_MUTEX_RECURSIVE, which tundra is dependent upon).
 This change allows tundra to be built without trouble on a vanilla
 Debian 6.0 installation. It shouldn't cause problems with non-GNU
 libc implementations such as OSX or the BSDs, and it is forward
 compatible with newer versions of glibc.