Fix scanning of Fortran sources created during configuration

mesonbuild/backend/ninjabackend.py

@@ -1693,12 +1693,13 @@ rule FORTRAN_DEP_HACK
         modre = re.compile(r"\s*module\s+(\w+)", re.IGNORECASE)
         module_files = {}
         for s in target.get_sources():
-            # FIXME, does not work for generated Fortran sources,
-            # but those are really rare. I hope.
+            # FIXME, does not work for Fortran sources generated by
+            # custom_target() and generator() as those are run after
+            # the configuration (configure_file() is OK)
             if not compiler.can_compile(s):
                 continue
-            filename = os.path.join(self.environment.get_source_dir(),
-                                    s.subdir, s.fname)
+            filename = s.absolute_path(self.environment.get_source_dir(),
+                                       self.environment.get_build_dir())
             with open(filename) as f:
                 for line in f:
                     modmatch = modre.match(line)