Update flags for external GCC.

Update flags for external GCC.

- The -L WORLDTMP/usr/lib is not needed as GCC is already adding in
  -L =/usr/lib internally with --sysroot.  It does not do this for
  header include paths though, thus passing -isystem =/usr/include is
  still needed.

For the forced libc++ usage:
- Use -isystem rather than -I for libc++ headers.
- Use -std=c++11 rather than gnu++11.
- Use -nostdinc++ to ensure GCC's headers don't leak in.

Sponsored by:	EMC / Isilon Storage Division