Add meson build system

Theres a couple of things that this meson build system does differently
than autotools. It doesn't use a config.h file, it just puts #defines on
the command line with -D. It also does all of the code generation in the
generated folder, simply because it's simpler to do that.

On my 2 core / 4 thread KBL system:

autotools (no ccache):
sh -c "./autogen.sh&& ./configure && make -j6 check"  44.74s user 6.70s system 145% cpu 35.269 total

autotools (warm ccache):
sh -c "./autogen.sh&& ./configure && make -j6 check"  32.86s user 4.22s system 129% cpu 28.580 total

meson (no ccache):
sh -c "meson build; ninja -C build test"  23.48s user 3.71s system 236% cpu 11.487 total

meson (warm ccache)
sh -c "meson build; ninja -C build test"  16.06s user 2.31s system 210% cpu 8.727 total