SERVER-13724 Add --disable-warnings-as-errors scons option

This change was prompted by attempts to build 2.6.0 in SLES 10 with
its system gcc version 4.1.2. It builds fine other than a sole
warning - src/third_party/boost/boost/thread/detail/thread.hpp:316:
warning: type attributes are honored only at type definition. It
should also help in cases where compilers are too new.