Make IWAD search paths compliant with XDG spec.

The XDG Base Directory Specification defines standard directories
that are searched for data files via the XDG_DATA_HOME and
XDG_DATA_DIRS environment variables:
http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html

To make Chocolate Doom a good citizen, use these directories to
determine some of the locations in which to look for IWAD files. This
includes /usr/share/games/doom, which is absorbed under the default
value of the XDG_DATA_DIRS variable. We also now support the alternative
~/.local/share/games/doom as a user-writeable path in the user's home
directory that doesn't require root privileges.

This is part of #597 (thanks to @chungy for the suggestion).

src/d_iwad.c

@@ -509,85 +509,126 @@ static GameMission_t IdentifyIWADByName(char *name, int mask)
     return mission;
 }
 
-//
-// Add directories from the list in the DOOMWADPATH environment variable.
-//
-
-static void AddDoomWadPath(void)
+// Add IWAD directories parsed from splitting a path string containing
+// paths separated by PATH_SEPARATOR. 'suffix' is a string to concatenate
+// to the end of the paths before adding them.
+static void AddIWADPath(char *path, char *suffix)
 {
-    char *doomwadpath;
-    char *p;
-
-    // Check the DOOMWADPATH environment variable.
-
-    doomwadpath = getenv("DOOMWADPATH");
-
-    if (doomwadpath == NULL)
-    {
-        return;
-    }
+    char *left, *p;
 
-    doomwadpath = M_StringDuplicate(doomwadpath);
-
-    // Add the initial directory
-
-    AddIWADDir(doomwadpath);
+    path = M_StringDuplicate(path);
 
     // Split into individual dirs within the list.
-
-    p = doomwadpath;
+    left = path;
 
     for (;;)
     {
-        p = strchr(p, PATH_SEPARATOR);
-
+        p = strchr(left, PATH_SEPARATOR);
         if (p != NULL)
         {
-            // Break at the separator and store the right hand side
+            // Break at the separator and use the left hand side
             // as another iwad dir
-
             *p = '\0';
-            p += 1;
 
-            AddIWADDir(p);
+            AddIWADDir(M_StringJoin(left, suffix, NULL));
+            left = p + 1;
         }
         else
         {
             break;
         }
     }
+
+    AddIWADDir(M_StringJoin(left, suffix, NULL));
+
+    free(path);
 }
 
+// Add standard directories where IWADs are located on Unix systems.
+// To respect the freedesktop.org specification we support overriding
+// using standard environment variables. See the XDG Base Directory
+// Specification:
+// <http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html>
+static void AddXdgDirs(void)
+{
+    char *env, *tmp_env;
+
+    // Quote:
+    // > $XDG_DATA_HOME defines the base directory relative to which
+    // > user specific data files should be stored. If $XDG_DATA_HOME
+    // > is either not set or empty, a default equal to
+    // > $HOME/.local/share should be used.
+    env = getenv("XDG_DATA_HOME");
+    tmp_env = NULL;
+
+    if (env == NULL)
+    {
+        char *homedir = getenv("HOME");
+        if (homedir == NULL)
+        {
+            homedir = "/";
+        }
+
+        tmp_env = M_StringJoin(homedir, "/.local/share", NULL);
+        env = tmp_env;
+    }
+
+    // We support $XDG_DATA_HOME/games/doom (which will usually be
+    // ~/.local/share/games/doom) as a user-writeable extension to
+    // the usual /usr/share/games/doom location.
+    AddIWADDir(M_StringJoin(env, "/games/doom", NULL));
+    free(tmp_env);
+
+    // Quote:
+    // > $XDG_DATA_DIRS defines the preference-ordered set of base
+    // > directories to search for data files in addition to the
+    // > $XDG_DATA_HOME base directory. The directories in $XDG_DATA_DIRS
+    // > should be seperated with a colon ':'.
+    // >
+    // > If $XDG_DATA_DIRS is either not set or empty, a value equal to
+    // > /usr/local/share/:/usr/share/ should be used.
+    env = getenv("XDG_DATA_DIRS");
+    if (env == NULL)
+    {
+        // (Trailing / omitted from paths, as it is added below)
+        env = "/usr/local/share:/usr/share";
+    }
+
+    // The "standard" location for IWADs on Unix that is supported by most
+    // source ports is /usr/share/games/doom - we support this through the
+    // XDG_DATA_DIRS mechanism, through which it can be overridden.
+    AddIWADPath(env, "/games/doom");
+}
 
 //
 // Build a list of IWAD files
 //
 
 static void BuildIWADDirList(void)
 {
-    char *doomwaddir;
+    char *env;
 
     if (iwad_dirs_built)
     {
         return;
     }
 
     // Look in the current directory.  Doom always does this.
-
     AddIWADDir(".");
 
     // Add DOOMWADDIR if it is in the environment
-
-    doomwaddir = getenv("DOOMWADDIR");
-
-    if (doomwaddir != NULL)
+    env = getenv("DOOMWADDIR");
+    if (env != NULL)
     {
-        AddIWADDir(doomwaddir);
-    }        
-
-    // Add dirs from DOOMWADPATH
+        AddIWADDir(env);
+    }
 
-    AddDoomWadPath();
+    // Add dirs from DOOMWADPATH:
+    env = getenv("DOOMWADPATH");
+    if (env != NULL)
+    {
+        AddIWADPath(env, "");
+    }
 
 #ifdef _WIN32
 
@@ -603,12 +644,7 @@ static void BuildIWADDirList(void)
     CheckSteamGUSPatches();
 
 #else
-
-    // Standard places where IWAD files are installed under Unix.
-
-    AddIWADDir("/usr/share/games/doom");
-    AddIWADDir("/usr/local/share/games/doom");
-
+    AddXdgDirs();
 #endif
 
     // Don't run this function again.