Extend the lifetime of the strings passed to posix_spawn_file_actions_addopen.

Thanks to Hal Finkel for finding the bug and for the initial patch.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187208 91177308-0d34-0410-b5e6-96231b3b80d8
Remove dead or useless header checks from cmake and autoconf

On Windows, this improves clean cmake configuration time on my
workstation from 1m58s to 1m32s, which is pretty significant.  There's
probably more that can be done here, but this is the low hanging fruit.

Eric volunteered to regenerate ./configure for me.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187209 91177308-0d34-0410-b5e6-96231b3b80d8