* gc.c: fix build failure on FreeBSD introduced by r43763.
  malloc_usable_size() is defined by malloc_np.h on FreeBSD.

* configure.in: check malloc.h and malloc_np.h.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43769 b2dd03c8-39d4-4d8f-98ff-823fe69b080e