Pass -n (do not emit comments) when saving mtree information for future
mergemaster(8) runs.

MFC after:	3 days
Approved by:	re (kib)
Emit the correct standard library dependency line for C++ programs.  In
the CLANG_IS_CC case, the default is now libc++.  Only use libstdc++ if
!CLANG_IS_CC or it was explicitly requested in CXXFLAGS.

Submitted by:	theraven
Approved by:	re (gjb)