ruby-pg project no longer uses 1.0.0a version but 1.0.1c instead. When
follow the warning instructions users will still fail to obtain the right
Allowing to provide an alternative version is good :-)
Define `FD_SETSIZE` unless it is already present in Ruby configuration.
This works around `FD_SETSIZE` not defined in some releases of
RubyInstaller and solve the segfault caused by mismatch value between
Ruby and the extension.