gnutls: fix build on sparc

gnutls source code uses the C++11 <atomic> functionality since
https://github.com/gnutls/gnutls/commit/7978a733460f92b31033affd0e487c86d66c643d,
which internally is implemented using the __atomic_*() gcc built-ins

On certain architectures, the __atomic_*() built-ins are implemented in
the libatomic library that comes with the rest of the gcc runtime. Due
to this, code using <atomic> might need to link against libatomic,
otherwise one hits build issues such as:

../lib/.libs/libgnutls.so: undefined reference to `__atomic_fetch_sub_4'

on an architecture like SPARC.

To solve this, a configure.ac check is added to know if we need to
link against libatomic or not. The library is also added to gnutls.pc.

Fixes:
 - http://autobuild.buildroot.org/results/6c749bd592ceffeacadd2ab570d127936cce64b2
 - http://autobuild.buildroot.org/results/30aa83d3cf3482af8a59250c196c85f4a278d343



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Tested-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>