dev-db/pgpool2: Bump to 3.7.5

dev-db/pgpool2: Bump to 3.7.5

Add a couple patches to fix a few options in
configure.ac. --without-pam no longer acts as if --with-pam was
given. Now uses pkg-config to get any special settings for
libmemcached rather than using fixed sub-directories
(${user_provided_path}/{include,lib,…}). Lastly, introduced AX_PTHREAD
to link in the appropriate threading library to fix bug 616302.

If libmemcached was built with USE=sasl which builds against
cyrus-sasl. cyrus-sasl builds against a threading library which
necessitates including the same threading in pgpool2 despite pgpool2
not being a threaded program.

pgpool2 is now able to build against LibreSSL as well as OpenSSL. The
ebuild now reflects this fact

Bug: https://bugs.gentoo.org/616302
Bug: https://bugs.gentoo.org/646888
Package-Manager: Portage-2.3.40, Repoman-2.3.9