Handle multiple substitutions in eval_custom_target_command()

Handle command arguments which contain multiple substitutions correctly
in Backend.eval_custom_target_command()

In particular, gnome.gtkdoc() makes arguments of the form '--cflags
-I@SOURCE_ROOT@ -I@BUILD_ROOT' (where these arguments are then passed
down to a compiler invocation)

Normally, these are subsequently made right by
NinjaBackend.replace_paths(), but if Backend.as_meson_exe_cmdline()
decides that the command needs to be pickled, that doesn't happen.

(Although having two places where this substitution might happen smells
really bad)

mesonbuild/backend/backends.py

@@ -1034,35 +1034,36 @@ class Backend:
             elif not isinstance(i, str):
                 err_msg = 'Argument {0} is of unknown type {1}'
                 raise RuntimeError(err_msg.format(str(i), str(type(i))))
-            elif '@SOURCE_ROOT@' in i:
-                i = i.replace('@SOURCE_ROOT@', source_root)
-            elif '@BUILD_ROOT@' in i:
-                i = i.replace('@BUILD_ROOT@', build_root)
-            elif '@DEPFILE@' in i:
-                if target.depfile is None:
-                    msg = 'Custom target {!r} has @DEPFILE@ but no depfile ' \
-                          'keyword argument.'.format(target.name)
-                    raise MesonException(msg)
-                dfilename = os.path.join(outdir, target.depfile)
-                i = i.replace('@DEPFILE@', dfilename)
-            elif '@PRIVATE_DIR@' in i:
-                if target.absolute_paths:
-                    pdir = self.get_target_private_dir_abs(target)
-                else:
-                    pdir = self.get_target_private_dir(target)
-                i = i.replace('@PRIVATE_DIR@', pdir)
-            elif '@PRIVATE_OUTDIR_' in i:
-                match = re.search(r'@PRIVATE_OUTDIR_(ABS_)?([^/\s*]*)@', i)
-                if not match:
-                    msg = 'Custom target {!r} has an invalid argument {!r}' \
-                          ''.format(target.name, i)
-                    raise MesonException(msg)
-                source = match.group(0)
-                if match.group(1) is None and not target.absolute_paths:
-                    lead_dir = ''
-                else:
-                    lead_dir = self.environment.get_build_dir()
-                i = i.replace(source, os.path.join(lead_dir, outdir))
+            else:
+                if '@SOURCE_ROOT@' in i:
+                    i = i.replace('@SOURCE_ROOT@', source_root)
+                if '@BUILD_ROOT@' in i:
+                    i = i.replace('@BUILD_ROOT@', build_root)
+                if '@DEPFILE@' in i:
+                    if target.depfile is None:
+                        msg = 'Custom target {!r} has @DEPFILE@ but no depfile ' \
+                              'keyword argument.'.format(target.name)
+                        raise MesonException(msg)
+                    dfilename = os.path.join(outdir, target.depfile)
+                    i = i.replace('@DEPFILE@', dfilename)
+                if '@PRIVATE_DIR@' in i:
+                    if target.absolute_paths:
+                        pdir = self.get_target_private_dir_abs(target)
+                    else:
+                        pdir = self.get_target_private_dir(target)
+                    i = i.replace('@PRIVATE_DIR@', pdir)
+                if '@PRIVATE_OUTDIR_' in i:
+                    match = re.search(r'@PRIVATE_OUTDIR_(ABS_)?([^/\s*]*)@', i)
+                    if not match:
+                        msg = 'Custom target {!r} has an invalid argument {!r}' \
+                              ''.format(target.name, i)
+                        raise MesonException(msg)
+                    source = match.group(0)
+                    if match.group(1) is None and not target.absolute_paths:
+                        lead_dir = ''
+                    else:
+                        lead_dir = self.environment.get_build_dir()
+                    i = i.replace(source, os.path.join(lead_dir, outdir))
             cmd.append(i)
         # Substitute the rest of the template strings
         values = mesonlib.get_filenames_templates_dict(inputs, outputs)