rabbitmq-c: openssl/popt sub-options don't work in static linking

rabbitmq-c: openssl/popt sub-options don't work in static linking

rabbitmq-c currently fails to build in a number of static linking
situations, due to two issues:

 - CMake FindOpenSSL module is buggy. Even though it uses pkg-config,
   it doesn't use the information returned by pkg-config, and
   therefore doesn't know about second order libraries that need be
   part of the link for static linking to succeed. Due to this, -lz is
   not passed, and therefore rabbitmq-c fails when linking against
   libssl/libcrypto. This issue has been reported to upstream CMake at
   https://gitlab.kitware.com/cmake/cmake/issues/16885.

 - popt might use libintl, but CMake doesn't know about that. For
   autotools based packages, we typically work around this by passing
   LIBS=, but CMake apparently has no equivalent to LIBS=.

To workaround this, we only use the OpenSSL and Popt optional
dependencies in dynamic linking situations.

Fixes:

  http://autobuild.buildroot.net/results/798dbe5e5fd0463bb2066cb115656795144c327f/

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