sngrep: use pkg-config to discover OpenSSL to fix static linking

Building sngrep with OpenSSL support in static linking configurations
currently fails due to undefined symbols in the OpenSSL library. It's
the usual problem with using AC_CHECK_LIB() to discover libraries
instead of the pkg-config based PKG_CHECK_MODULES().

Therefore, this commit introduces a patch that switches to using
pkg-config to discover OpenSSL. A preliminary patch is needed, without
which appending to LIBS/CFLAGS doesn't work. Both patches have been
submitted upstream.

Fixes:

  http://autobuild.buildroot.net/results/911143de823b2c749ac0a59dfa06adb6ddd3de50/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>