Add meson.

Meson is a cross-platform build system designed to be both as fast and as
user friendly as possible. It supports many languages and compilers, including
GCC, Clang and Visual Studio. Its build definitions are written in a simple
non-turing complete DSL. The main design point of Meson is that every moment
a developer spends writing or debugging build definitions is a second wasted.
So is every second spent waiting for the build system to actually start
compiling code.

Submitted by: Ting-Wei Lan