synth_mbrola: Add support for XDG_DATA_DIRS

On non-FHS systems like NixOS, there is no `/usr`. Additionally,
Nix has immutable packages so mbrola cannot be installed to espeak-ng’s
`datadir`. Instead, packages are expected to find data under one of the paths
listed in `XDG_DATA_DIRS` environment variable, as defined in the XDG Base
Directory Specification:
https://specifications.freedesktop.org/basedir-spec/basedir-spec-0.6.html#variables

src/libespeak-ng/synth_mbrola.c

@@ -62,6 +62,16 @@ static MBROLA_TAB *mbrola_tab = NULL;
 static int mbrola_control = 0;
 static int mbr_name_prefix = 0;
 
+static const char *system_data_dirs(void)
+{
+	// XDG Base Directory Specification
+	// https://specifications.freedesktop.org/basedir-spec/basedir-spec-0.6.html#variables
+	const char *dirs = getenv("XDG_DATA_DIRS");
+	if (dirs)
+		return dirs;
+	return "/usr/local/share:/usr/share";
+}
+
 espeak_ng_STATUS LoadMbrolaTable(const char *mbrola_voice, const char *phtrans, int *srate)
 {
 	// Load a phoneme name translation table from espeak-ng-data/mbrola
@@ -88,29 +98,48 @@ espeak_ng_STATUS LoadMbrolaTable(const char *mbrola_voice, const char *phtrans,
 	sprintf(path, "%s/mbrola/%s", path_home, mbrola_voice);
 #if PLATFORM_POSIX
 	// if not found, then also look in
-	//   usr/share/mbrola/xx, /usr/share/mbrola/xx/xx, /usr/share/mbrola/voices/xx
+	//   $data_dir/mbrola/xx, $data_dir/mbrola/xx/xx, $data_dir/mbrola/voices/xx
+	char *data_dirs = strdup(system_data_dirs());
+	char *data_dir = strtok(data_dirs, ":");
+	bool found = false;
 	if (GetFileLength(path) <= 0) {
-		sprintf(path, "/usr/share/mbrola/%s", mbrola_voice);
-
-		if (GetFileLength(path) <= 0) {
-			sprintf(path, "/usr/share/mbrola/%s/%s", mbrola_voice, mbrola_voice);
-
-			if (GetFileLength(path) <= 0)
-				sprintf(path, "/usr/share/mbrola/voices/%s", mbrola_voice);
-
-			// Show error message
-			if (GetFileLength(path) <= 0) {
-				fprintf(stderr, "Cannot find MBROLA voice file '%s' in neither of paths:\n"
-						" - /usr/share/mbrola/%s\n"
-						" - /usr/share/mbrola/%s/%s\n"
-						" - /usr/share/mbrola/voices/%s\n"
-						"Please install necessary MBROLA voice!\n",
-						mbrola_voice, mbrola_voice, mbrola_voice, mbrola_voice, mbrola_voice);
-				// Set path back to simple name, otherwise it shows misleading error only for
-				// last unsuccessfully searched path
-				sprintf(path, "%s", mbrola_voice);
+		while(data_dir) {
+			sprintf(path, "%s/mbrola/%s", data_dir, mbrola_voice);
+			if (GetFileLength(path) > 0) {
+				found = true;
+				break;
+			}
+
+			sprintf(path, "%s/mbrola/%s/%s", data_dir, mbrola_voice, mbrola_voice);
+			if (GetFileLength(path) > 0) {
+				found = true;
+				break;
+			}
+
+			sprintf(path, "%s/mbrola/voices/%s", data_dir, mbrola_voice);
+			if (GetFileLength(path) > 0) {
+				found = true;
+				break;
 			}
+
+			data_dir = strtok(NULL, ":");
 		}
+	} else {
+		found = true;
+	}
+	// Show error message
+	if (!found) {
+		fprintf(stderr, "Cannot find MBROLA voice file '%s' in neither of paths:\n"
+				" - $data_dir/mbrola/%s\n"
+				" - $data_dir/mbrola/%s/%s\n"
+				" - $data_dir/mbrola/voices/%s\n"
+				"for any data_dir in XDG_DATA_DIRS=%s\n"
+				"Please install necessary MBROLA voice!\n",
+				mbrola_voice, mbrola_voice, mbrola_voice, mbrola_voice, mbrola_voice,
+				system_data_dirs());
+		// Set path back to simple name, otherwise it shows misleading error only for
+		// last unsuccessfully searched path
+		sprintf(path, "%s", mbrola_voice);
 	}
 	close_MBR();
 #endif