Add flags needed for Clang to the build system.

This uses a variable CLANG that should be set to the Clang version. For
now, this has to be done manually (e.g. when invoking jam using jam
-sCLANG=34), but later, this will be auto-detected.

Work towards bug #10396
glibc: Remove a ++ that was accidentally added.

Thanks to PulkoMandy for noticing!

Work towards bug #10396