meson: Add options to not-build samples and tests.

[tsieprawski]

When using boxfort as a subproject, meson test runs boxfort's tests. I believe a typical developer developing their super project wants to run only own tests, not dependencies' ones.

Thankfully boxfort's tests do not have much of impact, so they at least clutter the view. :)

This PR adds -Dsamples and -Dtests options to enable/disable, like this:

meson setup -Dsamples=false -Dtests=false build && cd build && meson test
# Or as a subproject
meson setup -Dboxfort:samples=false -Dboxfort:tests=false build && cd build && meson test

See the attached asciinema cast: boxfort.cast.zip

[hydrapolic]

Yes please, this will also help packaging.