configure: allow passing extra libs per-frontend

configure: allow passing extra libs per-frontend

When building statically, it might be necessary to pass extra
libraries to some frontends. For example:
  nconf frontend, using ncursesw
  ncursesw is linked against libgpm

Because static dependencies do not follow (no they don't even
with libtool's .la libraries!), we have to set them manually.

Recognise xxx_LIBS as a list of extra libraries (in the form
-lyyy) to pass to frontend xxx.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>