Make sysctl name2oid shared-locked as well.

This is a follow-up to r273401.
The dependencies are computed with CC even if sources are C++, when building
when building with an external gcc, we want to be able to pass the path to
the libc++ headers so dependencies are correctly computed for C++ source files.
Add a DEPFLAGS for that purpose

Reviewed by:	imp
When using an external toolchain note that gcc 4.8+ supports C++11

Submitted by:	imp
Always use libc++ as the default c++ stack when building with an external gcc 4.8+
While here disable building gcc from base when using gcc 4.8+

Reviewed by:	imp