qucs-0.0.18.131128 failed to build under FreeBSD. In particular, the Qt
libraries are not found. Searching for them appeared to be originally intended
but then disabled in configure.ac, so all this could only work if Qt was
installed in a path where the linker would search for its libraries anyway.
I extended the commented-out library search in configure.ac, and made it work.
There are a couple of related minor changes. (I wonder whether the support
for FreeBSD 4's odd pthread handling is really worth keeping. FreeBSD 4.x is
really old now, and I doubt anyone would still want to use such a machine to
run things like Qucs on it.)