Fix compilation with static libcap and shared gnutls

Fix compilation with static libcap and shared gnutls

Commit c06ed219c47098f34485d408410b6ecc94a40877 has broken
compilation with a static libcap and a shared gnutls.
This results in a build failure on init_lxc_static if gnutls is
a shared library as init_lxc_static is built with -all-static option
(see src/lxc/Makefile.am) and AC_CHECK_LIB adds gnutls to LIBS.

This commit fix the issue by removing default behavior of AC_CHECK_LIB
and handling manually GNUTLS_LIBS and HAVE_LIBGNUTLS

Fixes:
 - http://autobuild.buildroot.net/results/b655d6853c25a195df28d91512b3ffb6c654fc90

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>