Use CPPFLAGS for -I, not CFLAGS or CXXFLAGS

CPPFLAGS is for the C Pre-Processor, and should be used for -I flags. In
a couple cases (nping, nmap) this was resulting in duplicate -Ilibpcap
arguments.
Re-autoconf the configure scripts (using 2.69)
Let nsock's configure take the same --with-libpcap as Nmap does