Revert r297516 - Respect CMAKE_INSTALL_MANDIR for sphinx generated manpages

When CMAKE_INSTALL_MANDIR isn't defined it ends up attempting to install
the man pages under "/man1" and we really don't want to accidentally install
stuff at the filesystem root.

llvm-svn: 297545
diff --git a/llvm/cmake/modules/AddSphinxTarget.cmake b/llvm/cmake/modules/AddSphinxTarget.cmake
index 2c836e4..3456b53 100644
--- a/llvm/cmake/modules/AddSphinxTarget.cmake
+++ b/llvm/cmake/modules/AddSphinxTarget.cmake
@@ -51,7 +51,7 @@
         # FIXME: We might not ship all the tools that these man pages describe
         install(DIRECTORY "${SPHINX_BUILD_DIR}/" # Slash indicates contents of
                 COMPONENT "${project}-sphinx-man"
-                DESTINATION ${CMAKE_INSTALL_MANDIR}/man1)
+                DESTINATION share/man/man1)
 
       elseif (builder STREQUAL html)
         string(TOUPPER "${project}" project_upper)