Respect CMAKE_INSTALL_MANDIR for sphinx generated manpages

This is a re-work of r297516, which was reverted in r297545.

https://reviews.llvm.org/D30906


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@299547 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed