docs: Allow building man pages without the reference documentation

Now that the reference documentation uses gi-docgen, it's more troublesome to generate in less standard build scenarios like cross-compiling.

In distributions like Debian, reference documentation is generally packaged separately (in libglib2.0-doc in Debian's case), but man pages are generally packaged alongside the executables themselves (in the libglib2.0-bin and libglib2.0-dev-bin packages, in Debian's case). We can exclude the reference documentation when cross-compiling, but ideally we would like the man pages to still be built, so that a cross-compiled libglib2.0-bin or libglib2.0-dev-bin package has the same content as a native build.