Support LINGUAS for gnome.yelp() languages

Listing all languages inside meson.build for the Yelp-based manual
localisation is error-prone, and it also requires parsing and modifying
Meson files from external tools.

Just like we do for i18n, we can use an ancillary `LINGUAS` file in the
help source sub-directory to list all the help languages.

mesonbuild/scripts/yelphelper.py

@@ -29,6 +29,22 @@ parser.add_argument('--media', dest='media', default='')
 parser.add_argument('--langs', dest='langs', default='')
 parser.add_argument('--symlinks', type=bool, dest='symlinks', default=False)
 
+def read_linguas(src_sub):
+    # Syntax of this file is documented here:
+    # https://www.gnu.org/software/gettext/manual/html_node/po_002fLINGUAS.html
+    linguas = os.path.join(src_sub, 'LINGUAS')
+    try:
+        langs = []
+        with open(linguas) as f:
+            for line in f:
+                line = line.strip()
+                if line and not line.startswith('#'):
+                    langs += line.split()
+        return langs
+    except (FileNotFoundError, PermissionError):
+        print('Could not find file LINGUAS in {}'.format(src_sub))
+        return []
+
 def build_pot(srcdir, project_id, sources):
     # Must be relative paths
     sources = [os.path.join('C', source) for source in sources]
@@ -108,6 +124,9 @@ def run(args):
     build_subdir = os.path.join(os.environ['MESON_BUILD_ROOT'], options.subdir)
     abs_sources = [os.path.join(src_subdir, 'C', source) for source in sources]
 
+    if not langs:
+        langs = read_linguas(src_subdir)
+
     if options.command == 'pot':
         build_pot(src_subdir, options.project_id, sources)
     elif options.command == 'update-po':

test cases/frameworks/13 yelp/help/LINGUAS

@@ -0,0 +1,2 @@
+de
+es

test cases/frameworks/13 yelp/help/meson.build

@@ -13,3 +13,9 @@ gnome.yelp('meson-symlink',
    symlink_media: true,
    languages: ['de', 'es'],
 )
+
+gnome.yelp('meson-linguas',
+   sources: 'index.page',
+   media: 'media/test.txt',
+   symlink_media: false,
+)

test cases/frameworks/13 yelp/installed_files.txt

@@ -10,3 +10,9 @@ usr/share/help/es/meson-symlink/media/test.txt
 usr/share/help/es/meson-symlink/index.page
 usr/share/help/de/meson-symlink/index.page
 usr/share/help/de/meson-symlink/media/test.txt
+usr/share/help/C/meson-linguas/index.page
+usr/share/help/C/meson-linguas/media/test.txt
+usr/share/help/es/meson-linguas/media/test.txt
+usr/share/help/es/meson-linguas/index.page
+usr/share/help/de/meson-linguas/index.page
+usr/share/help/de/meson-linguas/media/test.txt