build: add an option to not build man pages
When cross-compiling, the compiled binaries cannot be run on the build machine, unless one defines a helper (e.g. qemu) in the meson configuration, which is most of the time undesirable.
If help2man is installed on the build machine, then the build would fail because of this, so add an option to disable generating the man pages.
With later versions of meson, it will be possible to automatically detect this condition.
Based on the same fix done in libmbim by Nicolas Cavallari, see libmbim@792af73e
Fixes #75 (closed)