Set variable if extensions are supported.
Add PG_SHARE_PATH.
Fix extension version.
Add Cmake script to build the main extension file.

It reads in pgagent.sql, removes the BEGIN TRANSACTION and COMMIT TRANSACTION
lines, and writes out pgagent--$VERSION.sql. This only happens of Postgres
includes extension support. The file is also instaled into the Postgres
extension directory, along with any files matching `sql/*--*.sql`, which ought
to include any future upgrade files.