various win32 fixes

various win32 fixes
 - fixes that silence VC noises about dup exports, non-default libs, and
   unsupported *.def file directives
 - s/inplace/inplace_label/ malloc.c
 - update Config{usemymalloc} based on d_mymalloc
 - export Perl_*Vars
 - fix makefiles to not build miniperl.exe twice, and to make it properly
   when defaults are changed

p4raw-id: //depot/perl@1129