add Meson build system

Dylan Baker requested to merge dbaker/libxau:meson into master

I've chosen to not generate a config.h file with Meson. This makes using libXau as a subproject safer, since we don't have to worry about conflicting config.h files, plus there are only a couple of declarations that are actually used. This also saves some configure time in writing out an additional file.

Edited by Dylan Baker

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