fix configure PSQL_*= being ignored

make qtConfLibrary_psqlEnv() fall back to qtConfLibrary_inline() if
$PSQL_LIBS is not set.

Task-number: QTBUG-59521
Change-Id: Ie293e8bfaa3e113ede166243b345833973cc66f8
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

src/sql/configure.pri

@@ -36,6 +36,9 @@ defineTest(qtConfLibrary_psqlEnv) {
     !isEmpty(PSQL_LIBS) {
         $${1}.libs = $$PSQL_LIBS
         export($${1}.libs)
+    } else {
+        !qtConfLibrary_inline($$1): \
+            return(false)
     }
     return(true)
 }