package/postgresql: add cflags_sl option to pg_config

package/postgresql: add cflags_sl option to pg_config

Some external packages call pg_config to determine the installed
PostgreSQL cflags_sl option. Add this output to Buildroots own
pg_config, so these packages correctly compile.

Default value is defined at src/template/linux as:

Extra CFLAGS for code that will go into a shared library
CFLAGS_SL="-fPIC"

Signed-off-by: Maxim Kochetkov <fido_max@inbox.ru>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>