swift234

[Un-revert][Glibc] Configure modulemap for target, not host

This reverts commit f2154ee94d, which reverted 04e1cd5bda. The original
commit needed to be reverted because of an issue in which install
targets were added to OS X builds that did not target Linux. This
addresses that issue by guarding all the Linux-only CMake logic with a
check for the SDK being built.