Don't build static libraries on Cygwin

Don't build static libraries on Cygwin

Cygwin has been building and linking against static libraries. Although
a bug this has been relatively harmless until now, when this has caused
errors due to changes in the way we build certain libraries. So this
patch makes things work the way we always intended, namely that we would
link against the dynamic libraries (cygpq.dll etc.) and just not build
the static libraries. The downstream packagers have been doing this for
some time, so this just aligns with their practice.

Extracted from a patch by Marco Atzeri, with a suggestion from Tom Lane.

Discussion: https://postgr.es/m/1056.1538235347@sss.pgh.pa.us