build: move cscope rules to ninja

Makefile

@@ -8,8 +8,3 @@ build.ninja: configure.py
 
 clean:
 	rm -rf $(out)
-
-cscope:
-	find -name '*.[chS]' -o -name "*.cc" -o -name "*.hh" | cscope -bq -i-
-	@echo cscope index created
-.PHONY: cscope

configure.py

@@ -120,4 +120,9 @@ with open(buildfile, 'w') as f:
           command = python3 configure.py $configure_args
           generator = 1
         build build.ninja: configure | configure.py
-        '''))
+        rule cscope
+            command = find -name '*.[chS]' -o -name "*.cc" -o -name "*.hh" | cscope -bq -i-
+            description = CSCOPE
+        build cscope: cscope
+        default {modes_list}
+        ''').format(modes_list = ' '.join(build_modes), **globals()))