configure: fully handle --with-libpcap-* options
configure: fully handle --with-libpcap-* options

Configure options --with-libpcap-includes=DIR and
--with-libpcap-libraries=DIR were defined and used for the compilation
of tools, but they were ignore for the configure checks.

Fix bug #1320442 reported by Audric.

didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc