Build system: Improve detection of the iptc library.

Build system: Improve detection of the iptc library.

When checking for the iptc headers and data types, the configure script
added the kernel directory to the CFLAGS. Later, when actually building
the iptables plugin, the CFLAGS were left untouched.

At least on Debian, the “real” kernel headers are not required – the
libc versions in /usr/include/linux are sufficient. The usage of
KERNEL_DIR has therefore been removed from the iptc checks.

In addition, an directory specified by “--with-libiptc=/path” is no
longer added to the global CFLAGS but rather to the iptables specific
CPPFLAGS.

Hopefully this resolved build problems on various platforms.