guile: fix static-only build

guile uses bdwgc, which is already compiled with GC_NO_DLOPEN in
static-only scenarios. However, the bdwgc headers continue to include
<dlfcn.h> if used with GC_NO_DLOPEN undefined.

Therefore, to make guile link properly in a static-only scenario, we
have to build it with GC_NO_DLOPEN defined.

Fixes:

  http://autobuild.buildroot.net/results/1bf/1bf73dd4c28d6607e8c24d3abf259b8032712a02/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>