Add CMake EXPORT_SYMBOLS option and enable for FDW build

[senderista]

This is kind of a hack, and I didn't try to understand exactly which symbols needed to be exported for the FDW to work, but at least it's not intrusive on the FDW itself at all.

The FDW tests still fail, but at least the FDW lib is loaded successfully:

sudo -u postgres psql
postgres=# create extension gaia_fdw;
ERROR:  Error opening COW-SE session.
HINT:  Exception: 'Connect failed! - Connection refused'.

[LaurentiuCristofor]

Thanks for the fix!

[simone-gaia]

LGTM