Use PG_MODULE_MAGIC_EXT in PostgreSQL 18 and later

The PG_MODULE_MAGIC_EXT macro was added in PostgreSQL 18 and makes it
possible to see which version of the library is actually loaded using
pg_get_loaded_modules().

This commit also moves the use of PG_MODULE_MAGIC to a less awkward
place. While it is not 100% connected to the version functions it is
much more connect to them than to Djikstra.

Author: jeltz

src/dijkstra/dijkstra.c

@@ -37,8 +37,6 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 #include "c_types/path_rt.h"
 #include "process/shortestPath_process.h"
 
-PG_MODULE_MAGIC;
-
 PGDLLEXPORT Datum _pgr_dijkstra_v4(PG_FUNCTION_ARGS);
 PG_FUNCTION_INFO_V1(_pgr_dijkstra_v4);
 

src/version/version.c

@@ -29,6 +29,12 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 #include "c_common/postgres_connection.h"
 #include "./version.h"
 
+#ifdef PG_MODULE_MAGIC_EXT
+PG_MODULE_MAGIC_EXT(.name = "pgrouting", .version = PROJECT_VERSION);
+#else
+PG_MODULE_MAGIC;
+#endif
+
 #define UNUSED(x) (void)(x)
 
 PGDLLEXPORT Datum _pgr_boost_version(PG_FUNCTION_ARGS);