- Restructured and added a few comments here and there.

- Made macros of couple of tests previously littering configure.in
  (e.g. PHP_PROG_AWK, PHP_PROG_LEX, PHP_PROG_BISON..)
- Moved all PHP_SUBST() calls into their respective macros.
  (no point adding empty entries in Makefile if macro is not used)

Author: foobar