Fix Ruby bindings build on Mac OS X and FreeBSD.

Fix Ruby bindings build on Mac OS X and FreeBSD.

Using "\s*" on BSD sed doesn't work the same way as on GNU sed, thus
yielding e.g. " 3.0.8" instead of "3.0.8" in a ./configure check, which
later prevents the Ruby bindings from being built.

Fix this by simply using exactly one space in the sed invocation, since
swig seems to always use just one space there anyway.

Tested on Mac OS X, FreeBSD, and Linux.

This closes bug #800.