meson: support compiling as subproject
This is more of an RFC due to the limitations below.
The idea is to allow QEMU to build libslirp as a subproject. The main change is to skip installing devel files if compiling as a subproject, including the library if it is static; the parent project can specify that using 'default_library=static'.
Unfortunately, meson subprojects do not support add_dist_script, so we cannot generate the .tarball-version file at "meson dist" time. The ugly solution is to require a subproject option to be specified by the parent project.