configure: Add configure script used to detect NaCl

In order to use an installation of NaCl provided by the distro or by
previous manual installation, provide a configure script which checks
for nacl's presence.

Also check of libnl-genl headers. More checks of the other libraries
depended on by netsniff-ng as well as checks for necessary kernel
headers should be added here too.

The configure script is preliminary and has not yet extensively been
tested on multiple systems.

The concept was inspired by the configure script of trinity and
iproute2.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
gitignore: Ignore generated Config

Ignore the build config generated by the configure script.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
configure: Check for tpacket, ncurses, libgeoip

Check for some more libraries we make use of in the configure step.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
configure: Add linker flags for ncursestest to correct position

Newer versions of GCC expect the linker flags after the object code, so
move the ouput of 'pkg-config --libs ncurses' there to not make the test
fail even if ncurses is present on the system.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
configure: Check for presence of pkg-config and ccache binaries

Check whether pkg-config and ccache are in the PATH. A missing
pkg-config will cause the configure script to abort with an error. The
presence of ccache is written to Config and used by the Makefile to
conditionally make use of it.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
configure: Improve check for libnl/libnl-genl

Actually call some functions in the check program so they will need to
get linked.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Merge branch 'config'

Pull in the configure script. From now on, before calling 'make' the
'./configure' script will need to get called.

This is no autotools generated script, but manually written to check for
netsniff-ng's dependencies.