don't load modules if they are built in

Only load modules if libvips has been built as a set of modules.
Otherwise can can end up loading operations twice.

ChangeLog

@@ -1,5 +1,6 @@
 14/8/20 started 8.11.2
 - better libdir guessing [remi]
+- don't load modules if we're built without modules
 
 14/8/20 started 8.11.1
 - add more example code to C docs

configure.ac

@@ -459,6 +459,7 @@ else
   if test x"$gmodule_supported" = x"true"; then
     gmodule_supported_flag=yes
     gmodule_with_flag='module'
+    AC_DEFINE(ENABLE_MODULES,1,[define to enable loadable module support.])
     AC_MSG_RESULT([yes])
   else
     AC_MSG_RESULT([no])

libvips/iofuncs/init.c

@@ -536,11 +536,17 @@ vips_init( const char *argv0 )
 	vips_mosaicing_operation_init();
 	vips_g_input_stream_get_type(); 
 
+#ifdef ENABLE_MODULES
 	/* Load any vips8 modules from the vips libdir. Keep going, even if
 	 * some modules fail to load. 
+	 *
+	 * Only do this if we have been built as a set of loadable
+	 * modules, or we might try loading an operation into a library that
+	 * already has that operation built in.
 	 */
 	(void) vips_load_plugins( "%s/vips-modules-%d.%d", 
 		libdir, VIPS_MAJOR_VERSION, VIPS_MINOR_VERSION );
+#endif /*ENABLE_MODULES*/
 
 #if ENABLE_DEPRECATED
 	/* Load any vips8 plugins from the vips libdir.