This allows someone without pkg-config in their PATH to set where
pkg-config is located.

We depend on pkg-config, but we weren't explicitly testing for it, so
the absence of pkg-config would cause the sqlite test to fail, which
was confusing.

This makes portable the fix from commit
28ee7736.

FS-6475 --resolve

CID: 1211948 fix memory leak of decoded iks message in function parse_plain_auth_message in mod_rayo