Revert "Do version AST merging after all parsing has finished"

This reverts commit f884e3f778f2b959b628b7faaeb9216844f883be.

Conflicts:
	fbfrog.bas
Remove -noautoextern option

The automated Extern block generation is always useful; there doesn't seem
to ever be a case where you'd want to disable it.
Remove -pragmaonce option, always use it automatically
Remove frog.commandline variable (unused)
cpp: Support variadic macros