Make gettext targets no-ops if gettext is not installed. Closes: #821.

docs/markdown/snippets/nogettext.md

@@ -0,0 +1,7 @@
+## Gettext targets are ignored if `gettext` is not installed
+
+Previously the `i18n` module has errored out when `gettext` tools are
+not installed on the system. Starting with this version they will
+become no-ops instead. This makes it easier to build projects on
+minimal environments (such as when bootstrapping) that do not have
+translation tools installed.

mesonbuild/modules/i18n.py

@@ -15,7 +15,7 @@
 import shutil
 
 from os import path
-from .. import coredata, mesonlib, build
+from .. import coredata, mesonlib, build, mlog
 from ..mesonlib import MesonException
 from . import ModuleReturnValue
 from . import ExtensionModule
@@ -55,8 +55,18 @@ PRESET_ARGS = {
     ]
 }
 
+
 class I18nModule(ExtensionModule):
 
+    nogettext_warning_printed = False
+
+    @classmethod
+    def nogettext_warning(cls):
+        if not cls.nogettext_warning_printed:
+            mlog.warning('Gettext not found, all translation targets will be ignored.')
+            cls.nogettext_warning_printed = True
+        return ModuleReturnValue(None, [])
+
     @staticmethod
     def _get_data_dirs(state, dirs):
         """Returns source directories of relative paths"""
@@ -67,6 +77,8 @@ class I18nModule(ExtensionModule):
     @FeatureNewKwargs('i18n.merge_file', '0.51.0', ['args'])
     @permittedKwargs(build.CustomTarget.known_kwargs | {'data_dirs', 'po_dir', 'type', 'args'})
     def merge_file(self, state, args, kwargs):
+        if not shutil.which('xgettext'):
+            return self.nogettext_warning()
         podir = kwargs.pop('po_dir', None)
         if not podir:
             raise MesonException('i18n: po_dir is a required kwarg')
@@ -120,7 +132,7 @@ class I18nModule(ExtensionModule):
         if len(args) != 1:
             raise coredata.MesonException('Gettext requires one positional argument (package name).')
         if not shutil.which('xgettext'):
-            raise coredata.MesonException('Can not do gettext because xgettext is not installed.')
+            return self.nogettext_warning()
         packagename = args[0]
         languages = mesonlib.stringlistify(kwargs.get('languages', []))
         datadirs = self._get_data_dirs(state, mesonlib.stringlistify(kwargs.get('data_dirs', [])))