[win32] various changes to make win32 build under the new Configure & co.

[win32] various changes to make win32 build under the new Configure & co.
 - added byterun.c to core C build
 - makefile.mk now has a regen_config_h target to quickly update config_H.[bgv]c
   after adding new variables to config.[bgv]c
 - sig_name_init now has only the valid signals
 - we now have $Config{usethreads}
 - tested under the two commercial compilers w/ and w/o usethreads

p4raw-id: //depot/win32/perl@590