SERVER-9221 fix linkage of libmongoclient.so

- gsasl was not linked against the lib when using --use-sasl-client and --sharedclient
- two unneeded cpp files caused linker errors

Conflicts:
	src/SConscript.client
	src/mongo/SConscript
SERVER-9221 Fixed broken build of C++ driver's shared library

* Don't exclude assert macroses when building the client library
  - they are needed to build it, but will be successfully excluded then
  by undef_macros.h in dbclient.h
* Added missing #include "mongo/util/assert_util.h"