PGSQL and POD_SQL: don't include pg_config.h

Even if that header file is available, we better consider it private,
and don't include it.  The information about whether SSL support is
enabled is now missing (`USE_(OPEN)SSL`), and it seems there is no
alternative way to get it (`PQinitSSL()` is always defined), so we
remove it from the PHP info.  Furthermore, the `PG_VERSION` and
`PG_VERSION_STR` macros are no longer available, but as of libpq 9.1
there is `PQlibVersion()` which allows us to construct `PG_VERSION` in
a most likely backwards compatible manner.  The additional information
available through `PG_VERSION_STR` is lost, though, so we define
`PGSQL_LIBPQ_VERSION_STR` basically as alias of `PGSQL_LIBPQ_VERSION`,
and deprecate it right away.

Since we are now requiring at least libpq 9.1, we can remove some
further compatibility code and additional checks.

Regarding the raised requirements: official support for PostGreSQL 9.0
ended on 2015-10-08, and even CentOS 7 already has PostGreSQL 9.2, so
this is not supposed to be too much of an issue.

ext/pdo_pgsql/php_pdo_pgsql_int.h

@@ -107,4 +107,6 @@ enum pdo_pgsql_specific_constants {
 php_stream *pdo_pgsql_create_lob_stream(zval *pdh, int lfd, Oid oid);
 extern const php_stream_ops pdo_pgsql_lob_stream_ops;
 
+void pdo_libpq_version(char *buf, size_t len);
+
 #endif /* PHP_PDO_PGSQL_INT_H */