Include libpq development packages in the repos. (#5667)

While we're at it:
- Automatically find the latest PostgreSQL installation on EL/Fedora
- Use libpq5 on EL/Fedora, not postgresql-libs.

Fixes #5650

pkg/debian/build.sh

@@ -122,7 +122,7 @@ fakeroot dpkg-deb --build "${METAROOT}" "${DISTROOT}/${APP_NAME}_${APP_LONG_VERS
 
 # Get the libpq package
 pushd "${DISTROOT}" 1> /dev/null
-apt-get download libpq5
+apt-get download libpq5 libpq-dev
 popd 1> /dev/null
 
 echo "Completed. DEBs created in ${DISTROOT}."