CI: Use libpcap.a on MATRIX_BUILD_LIBPCAP=yes. [skip appveyor]

CI: Use libpcap.a on MATRIX_BUILD_LIBPCAP=yes. [skip appveyor]

The only way to pass tests on OpenBSD is to link with the upstream
libpcap using ../libpcap/libpcap.a (hence not using CMake) because
-lpcap always results in linking with OpenBSD /usr/lib/libpcap.so
regardless of the libpcap.a and libpcap.so that build.sh installs under
/tmp/local/.

Work around by not cleaning in ../libpcap on MATRIX_BUILD_LIBPCAP=yes,
so Autoconf can pick the static library up. On MATRIX_BUILD_LIBPCAP=no
do the cleaning so it cannot.

(cherry picked from commit 40733327128628b8b0db270c4220beaaad1d6e02)