Makefile.sub: fix mswin build

* win32/Makefile.sub (config.h): fix mswin build, also VC has time.h
  and struct timeval.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39779 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
test_bignum.rb: refine test

* test/ruby/test_bignum.rb (test_interrupt_during_to_s): rename and
  merge with assert_interrupt, separate Bignum power from to_s, get
  rid of busy loop, and skip if the conversion is too fast.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39780 b2dd03c8-39d4-4d8f-98ff-823fe69b080e