New build system

[Alan DeKok]

  OK, I've fixed a bug with dependencies, where it wouldn't re-build a C
file if a header changed.  I've also added some "configure" support.  So
modules like rlm_pam, python, perl, etc. only get built if their
prerequisites are found.

  One benefit is that "make -j 8" now works.  On my quad-core system I
get this:

$ time make -j 8 clean all
...
real	0m4.357s
user	0m24.702s
sys	0m3.580s

  A complete build in less than 5 seconds.  Very nice!

  Alan DeKok.