Eliminated some warning that were pointed out by clang++. However, due
to some issue with template instantiations with namespace mixups
(well, that's a wild guess anyway), the way npm uses fpplib only
compiles with g++. So, make sure to use that for now, e.g. by setting
the CC and CXX environment variables before running cmake.

Also, the fpplib build setup had got out of synch with its directory
layout. Not that it matters so much for npm, but sooner or later
fpplib has to get updated to clang and a more recent yamlcpp.
note that clang does not work for the moment