Add a meson build system

[1480c1]

Allows for easy {cross-,}compilation for Windows and a lot of other systems

Adds a meson.build file that actually contains the directives for meson and the reversion.sh script written in basic python so it doesn't require a working sh

Also modifies unifdef.c to use <unifdef.h> so it's possible to use -I to influence where it decides to look for the header instead of looking relative to the file itself.

Currently, only supports building and installing unifdef$(EXE_SUFIX) and the manpages, but does not support the test, realclean, Changelong, unifdef.txt, and release targets yet as those will need additional work

to run a basic build,

meson builddir .
ninja -C builddir
sudo ninja -C builddir install

and that will produce ./builddir/unifdef and install /usr/local/bin/unifdef and the man pages

I am thinking of removing the sln file and the Makefile.mingw after I work on making sure there is nothing missing and after adding GitHub Actions