guile: fix libltdl, libgmp and libunistring detection

Explicitly pass the path to libtldl, libgmp and libunistring, so that
guile does not try to use host versions when available.

Fixes:

  http://autobuild.buildroot.org/results/056/056b7fcbf3b73a8d15ca635b000771cdc2d7fbf9/



And another similar problem with libgmp and libunistring.

[Thomas: added --with-libunistring-prefix, as suggested by Samuel
Martin.]

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Tested-by: Samuel Martin <s.martin49@gmail.com>