macros moved to extconf.h.
* ext/extmk.rb (extract_makefile, extmk): made RUBY_EXTCONF_H and
EXTSTATIC permanent.
* ext/{dbm,digest/*,socket,zlib}/extconf.rb: used $defs and $INCFLAGS.
* {bcc32,win32,wince}/Makefile.sub (COMPILE_C, COMPILE_CXX): added
$(INCFLAGS).
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@10193 b2dd03c8-39d4-4d8f-98ff-823fe69b080e