poppler: link with libatomic when needed

Following errors might occur on architectures required to use gcc libatomics:
sparc-buildroot-linux-uclibc/sysroot/lib/libatomic.so.1: error adding symbols: DSO missing from command line

This is often the case for sparcv8 32 bit toolchains.

See 55a9d6d5 and 03f6e005.

Fixes:
  http://autobuild.buildroot.net/results/64a96663a48ab644bc03c9a3ec2d6a644119dee6
  http://autobuild.buildroot.net/results/82e85e12478fc1972b70ad728ed7c1554920c9e3



Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>