if possible, execute child ruby with environment variables
LD_PRELOAD=libumem.so UMEM_OPTIONS="backend=mmap". With these
variables, freed memory is immediately returned to the OS.
[Bug #10020] [ruby-dev:48391]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46791 b2dd03c8-39d4-4d8f-98ff-823fe69b080e