Configure options

[jwt27]

Work in progress, pushing what I have so far:

First commit saves the user's $CFLAGS in the makefile, which I think is generally a good idea. Though there are some targets that produce multiple makefiles, not sure how practical it is for those.

Second commit then allows users to override the config.h macros via $CFLAGS. For example, to make a "release" version of the library for djgpp:

$ export CFLAGS='-O3 -DNDEBUG -DWANT_DEBUG=0 -DWANT_BSD_FATAL=0'
$ ./configur.sh djgpp
$ make -f djgpp.mak

This is just the easiest way to implement it, but maybe not the nicest to use. I'm looking into adding autoconf-style options like --enable-dhcp, and a --help that explains all of them.