Detection based on gnutls_session doesn't work because GnuTLS 2.x.y
comes with a compat.h that defines gnutls_session to gnutls_session_t.
Instead detect this based on LIBGNUTLS_VERSION_MAJOR. Move this from
configure/config.h to gnutls_1_0_compat.h and make sure that all users
include gnutls_1_0_compat.h properly.
Also fix header guard in gnutls_1_0_compat.h.
Use <config.h>, not "config.h", per autoconf documentation.
* Makefile.cfg (local-checks-to-skip) [sc_require_config_h]: Enable.
* .x-sc_require_config_h: New file, to list exempted files.
* Makefile.am (EXTRA_DIST): Add .x-sc_require_config_h.
* acinclude.m4, configure.in: ./configure option
--disable-stack-protector.
* src/gnutls_1_0_compat.h: Compatibility with GnuTLS 1.0.
* src/bridge.c: If no bridge ioctls, give an error at runtime.