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