config: fallback to /etc/xdg if XDG_CONFIG_DIRS is unset

Closes #1008

CHANGELOG.md

@@ -98,6 +98,8 @@
 * Underline cursor sometimes being positioned too low, either making
   it look thinner than what it should be, or being completely
   invisible (https://codeberg.org/dnkl/foot/issues/1005).
+* Fallback to `/etc/xdg` if `XDG_CONFIG_DIRS` is unset
+  (https://codeberg.org/dnkl/foot/issues/1008).
 
 
 ### Security

config.c

@@ -444,10 +444,12 @@ open_config(void)
         }
     }
 
-    /* Finally, try foot/foot.ini in all XDG_CONFIG_DIRS */
+    /* Finally, try foot/foot.ini in all XDG_CONFIG_DIRS, or /etc/xdg
+     * if unset */
     const char *xdg_config_dirs = getenv("XDG_CONFIG_DIRS");
     xdg_config_dirs_copy = xdg_config_dirs != NULL
-        ? strdup(xdg_config_dirs) : NULL;
+        ? strdup(xdg_config_dirs)
+        : strdup("/etc/xdg");
 
     if (xdg_config_dirs_copy != NULL) {
         for (char *save = NULL,

doc/foot.ini.5.scd

@@ -13,9 +13,10 @@ say if it needs to be reopened after any of the other sections.
 foot will search for a configuration file in the following locations,
 in this order:
 
-	- *XDG_CONFIG_HOME/foot/foot.ini*
-	- *~/.config/foot/foot.ini*
-	- *XDG_CONFIG_DIRS/foot/foot.ini*
+	- *XDG_CONFIG_HOME/foot/foot.ini* (defaulting to
+	  *~/.config/foot/foot.ini* if unset)
+	- *XDG_CONFIG_DIRS/foot/foot.ini* (defaulting to
+	  */etc/xdg/foot/foot.ini* if unset)
 
 # SECTION: main