WIP: Switch the buildsystem to Meson

The current autotools build system is generally working, it is sort of old, it comes with different languages, and adds a lot of autogenerated files for releases; it also has portability issues (see #28).

Since libspectre does not have a complex build need, it is possible to replace it with Meson. This solves a number of issues:

• only a simple language for the build system, no need for m4 + make
• easier to modify than autotools
• no more libtool
• no more generated files on releases; the release tarball is way smaller now: ~53K xz-compressed vs. ~287K xz-compressed from autotools (recompressed from the original gz-compressed which is ~424K)
• pkg-config file generated automatically
• less than half the lines of meson files vs the autotools-related files
• faster to configure and build than autotools

Checklist:

• builds fine
• works fine
  • I will do more checks
• ninja dist/meson dist works
• CI works
  • sadly Trusty is too old for Meson+ninja, switched to Xenial
• release stuff in Makefile.am ported
  • is it still needed/used?
• tested outside Linux (e.g. macOS or Windows)
  • cannot do on my own
• adapt the fuzz stuff in test, i.e. test/README and test/ossfuzz.sh; possibly also the Dockerfile hosted on the oss-fuzz repository

libspectre 0.2.9 was released few months ago, and there are many months between releases: so this seems a good moment to switch the build system, as there is plenty of time to test it and fix it.