build: Use Requires.private and Libs.private.
* configure.ac (LIBASSUAN_CONFIG_LIBS): Remove NETLIBS. * src/libassuan.pc.in: Distinguish static link. * tests/Makefile.am: Don't need to link NETLIBS.
- GnuPG-bug-id: T4624
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>