Remove the need to run `gtkdocize' when building from git; this should
be an operation done by the maintainer when modernizing the gtk-doc
setup (think of e.g. gettextize), no need to do it unconditionally.

This makes it easier for platforms without gtk-doc to build from git.

When trying to build with documentation enabled and gtk-doc isn't
found, we get some nice warnings and errors in the configure report:

    checking for gtk-doc... no
    configure: WARNING:
      You will not be able to create source packages with 'make dist'
      because gtk-doc >= 1.0 is not found.
    checking for gtkdoc-check... no
    checking for gtkdoc-check... no
    checking for gtkdoc-rebase... no
    checking for gtkdoc-mkpdf... no
    checking whether to build gtk-doc documentation... yes
    configure: error:
      You must have gtk-doc >= 1.0 installed to build documentation for
      ModemManager. Please install gtk-doc or disable building the
      documentation by adding '--disable-gtk-doc' to './configure'.

Files generated with gtkdocize (gtk-doc) 1.25.1.