boost.m4: do not issue useless -L/usr/local/lib

Yo!

This commit has fixed some issue I was having on ArchLinux.
I don't know how to check this in the test suite though.

Cheers!

Symptoms: on ArchLinux, some library linked with Boost thanks to
boost.m4 end up with the following errors:

vaucanson-git E: Insecure RPATH '/usr/local/lib' in file
('usr/lib/python3.4/site-packages/vcsn_cxx.so')
vaucanson-git E: Insecure RPATH '/usr/local/lib' in file
('usr/lib/vcsn/libvcsn.so.0.0.0')

The weird thing is that there is no reason for this dependency on
-L/usr/local/lib.  Careful reading of boost.m4 shows this is non
intended.  Indeed the empty library location ('') is tested first, but
following code is not ready to handle an empty path, which results in
various errors that are misunderstood as meaning that the libraries
are not there.  So the next succesfull location tried (which turns out
to be /usr/local/lib) will be "accepted", and uselessly propagated to
the linker.

* build-aux/boost.m4 (_BOOST_FIND_LIBS): Try first without any -L,
-rpath etc.