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>
(cherry picked from commit ed4cfbb7730b0172f1e503a279e214810eec049f)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/postgresql/pg_config

@@ -31,6 +31,10 @@ case "$1" in
   --cflags)
 	echo "@TARGET_CFLAGS@"
 	;;
+  --cflags_sl)
+	# defined at src/template/linux
+	echo "-fPIC"
+	;;
   --cc)
 	echo "@TARGET_CC@"
 	;;