Don't rebuild generated files based on file modification time
anymore, the action is now explicit. Replace "make touch"
with "make regen-all".

Changes:

* Remove "make touch", Tools/hg/hgtouch.py and .hgtouch
* Add a new "make regen-all" command to rebuild all generated files
* Add subcommands to only generate specific files:

  - regen-ast: Include/Python-ast.h and Python/Python-ast.c
  - regen-grammar: Include/graminit.h and Python/graminit.c
  - regen-opcode-targets: Python/opcode_targets.h

* Add PYTHON_FOR_REGEN variable
* pgen is now only built by by "make regen-grammar"
* Add $(srcdir)/ prefix to paths to source files to handle correctly
  compilation outside the source directory