plugins: Improve build with !HAVE_PLUGINS

Instead of not compiling plugins.c without HAVE_PLUGINS, we
should disable plugin support in a way that is functionally
the same as if the platform does not support it at runtime.

This reduces the number of ifdefs and allows sharing more utility
code for plugins.

wsutil/CMakeLists.txt

@@ -183,6 +183,7 @@ set(WSUTIL_COMMON_FILES
 	cpu_info.c
 	os_version_info.c
 	please_report_bug.c
+	plugins.c
 	privileges.c
 	regex.c
 	rsa.c
@@ -214,12 +215,6 @@ if(WIN32)
 	)
 endif()
 
-if(ENABLE_PLUGINS)
-	list(APPEND WSUTIL_COMMON_FILES
-		plugins.c
-	)
-endif()
-
 set(WSUTIL_FILES
 	${WMEM_FILES}
 	${WSUTIL_COMMON_FILES}

wsutil/plugins.c

@@ -236,7 +236,7 @@ DIAG_ON_PEDANTIC
 plugins_t *
 plugins_init(plugin_type_e type)
 {
-    if (!g_module_supported())
+    if (!plugins_supported())
         return NULL; /* nothing to do */
 
     GHashTable *plugins_module = g_hash_table_new_full(g_str_hash, g_str_equal, NULL, free_plugin);
@@ -342,7 +342,11 @@ plugins_cleanup(plugins_t *plugins)
 bool
 plugins_supported(void)
 {
+#ifndef HAVE_PLUGINS
+    return false;
+#else
     return g_module_supported();
+#endif
 }
 
 plugin_type_e