* gc.c: allow to tune growth of heap by environment variable
  RUBY_HEAP_SLOTS_GROWTH_FACTOR. patched by tmm1(Aman Gupta).
  [Feature #8015] [ruby-core:53131]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39746 b2dd03c8-39d4-4d8f-98ff-823fe69b080e