configure: Fix check for libnetfilter-conntrack

Correct the type to struct nf_conntrack and call a function to prevent
the variable from getting optimized away by the compiler and cause a
linker call. Also add the proper linker flag.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
configure: Call a function from zlib in check_zlib()

In order to cause the linker to get called and thus the check for the
presence of the library (not just the headers) to be performed, call
gzopen(). Also pass the proper linker flags to $CC.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>