Watcom makefiles overhaul.

- make both libcurl and curl makefiles use register calling convention
  (previously libcurl had stack calling convention).
- added include paths to the Watcom headers so its no longer required
  to set the environment vars for this.
- added -wcd=201 to supress compiler warning about unreachable code.
- use macros for all tools, and removed dependency on GNU tools like rm.
- make ipv6 and debug builds controlable via env vars and so make them
  optional instead of default.
- commented WINLDAPAPI and WINBERAPI since they broke with OW 1.8, and
  it seems they're not needed (anymore?).
- added rule for hugehelp.c.cvs so that it will be created when not
  already exist - this is required for building from a release tarball
  since there we have no hugehelp.c.cvs, thus compilation broke.
- removed C_ARG creation from lib/Makefile.Watcom and use CFLAGS
  directly as done too in src/Makefile.Watcom - this has the benefit
  that we will see all active cflags and defines during compile.
- added LINK-ARG to src/Makefile.Watcom in order to better control
  linker input.
- a couple of other minor makefile tweaks here and there ...
- added largefile support for Watcom builds to config-win32.h. Not yet
  tested if it really works, but should since Win32 supports it.
- added loaddll stuff to speed up builds if supported.