libpcap: Avoid openssl full library path

[rprasanth]

OPENSSL_LIBRARIES provide the entire path When I used the pcap-config to grep the LIBS in ppp build, it fails with below error, cannot find -l/path-to-/lib/libssl.so cannot find -l/path-to-/toolchain/lib/libcrypto.so

[guyharris]

This doesn't work with MinGW on Windows; see the failing AppVeyor build.