package/ipsec-tools: fix static build with openssl and atomic

Use pkg-config to find openssl dependencies such as lz or latomic

Fix static build on sparc v8 (even if there is no autobuilder failures
yet)

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>