meson: build summary
This roughly mirrors what we get from autotools. There's a few differences, though:
- The "exec_prefix" output has been dropped. Meson doesn't support this, so it makes no sense here.
- The "llvm-config" output has been dropped. Meson abstracts dependency discovery a bit more than our autotools build-system does, so it's not easy to get this information as-is.
- CFLAGS / CXXFLAGS / LDFLAGS works quite differently, and outputs quite a bit more flags than on Autotools.
- How we set defines works quite differently in our Meson build-system, and the result isn't quite the same. In particular, the DEFINES output has been dropped, to avoid having to refactor the code too much.