standard C++ runtime libraries.
we need to explicitly do this because ${CXX} only links against the
C++ runtime libraries when building an executable, not a shared
object. this results in executables linked against libraries linked by
${CXX} unable to resolve C++ symbols at runtime.
fixes packages which link shared libraries using ${CXX}, including
graphics/glu, databases/db4, and packages which use shared libraries
provided by these packages.
