Allow not-required not-found dependencies in subprojects

Closes https://github.com/mesonbuild/meson/issues/1474

mesonbuild/interpreter.py

@@ -1938,8 +1938,14 @@ class Interpreter(InterpreterBase):
         try:
             dep = self.subprojects[dirname].get_variable_method([varname], {})
         except KeyError:
-            raise InvalidCode('Fallback variable {!r} in the subproject '
+            if kwargs.get('required', True):
-                              '{!r} does not exist'.format(varname, dirname))
+                m = 'Fallback variable {!r} in the subproject {!r} does not exist'
+                raise DependencyException(m.format(varname, dirname))
+            # If the dependency is not required, don't raise an exception
+            mlog.log('Also couldn\'t find the dependency', mlog.bold(name),
+                     'in the fallback subproject',
+                     mlog.bold(os.path.join(self.subproject_dir, dirname)))
+            return None
         if not isinstance(dep, DependencyHolder):
             raise InvalidCode('Fallback variable {!r} in the subproject {!r} is '
                               'not a dependency object.'.format(varname, dirname))

test cases/common/95 dep fallback/meson.build

@@ -5,7 +5,11 @@ bob = dependency('boblib', fallback : ['boblib', 'bob_dep'], required: false,
 if not bob.found()
     error('Bob is actually needed')
 endif
+# boblib subproject exists, but sita_dep doesn't exist
+sita = dependency('sitalib', fallback : ['boblib', 'sita_dep'], required: false)
+# jimmylib subproject doesn't exist
 jimmy = dependency('jimmylib', fallback : ['jimmylib', 'jimmy_dep'], required: false)
+# dummylib subproject fails to configure
 dummy = dependency('dummylib', fallback : ['dummylib', 'dummy_dep'], required: false)
 
 gensrc_py = find_program('gensrc.py')