Join build makefiles together

Join build makefiles together

Changes:
- Joins build/build.mk and build/build2.mk files together since there
  isn't any practical reason for having two different files with the
  current build system.
- Makefile is now more portable. All special syntaxes are omitted, for
  example, a conditional assignment operators `?=`. This makes buildconf
  more useful on Solaris make derivative, so there is no longer need to
  override make with gmake: `MAKE=gmake ./buildconf`.
- Suppressing autoconf and autoheader warnings is not needed anymore
  with current build system. Instead, the option `-Wall` has been used
  when running `./buildconf --debug` to get more useful debug info
  about current M4.