scons: Add support for google protobuf building

This patch enables the use of protobuf input files in the build
process, thus allowing .proto files to be added to input. Each .proto
file is compiled using the protoc tool and the newly created C++
source is added to the list of sources.

The first location where the protobufs will be used is in the
capturing and replay of memory traces, involving the communication
monitor and the trace-generator state of the traffic generator. This
will follow in the next patch.

This patch does add a dependency on the availability of the BSD
licensed protobuf library (and headers), and the protobuf compiler,
protoc. These dependencies are checked in the SConstruct, similar to
e.g. swig. The user can override the use of protoc from the PATH by
specifying the PROTOC environment variable.

Although the dependency on libprotobuf and protoc might seem like a
big step, they add significant value to the project going
forward. Execution traces and other types of traces could easily be
added and parsers for C++ and Python are automatically generated. We
could also envision using protobufs for the checkpoints, description
of the traffic-generator behaviour etc. The sky is the limit. We could
also use the GzipOutputStream from the protobuf library instead of the
current GPL gzstream.

Currently, only the C++ source and header is generated. Going forward
we might want to add the Python output to support simple command-line
tools for displaying and editing the traces.

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