Per gripe from Japin Li.
Discussion: https://postgr.es/m/MEYP282MB16692171F13B5DF40DB768EEB6FCA@MEYP282MB1669.AUSP282.PROD.OUTLOOK.COM
# define the build and clean rules below.
#
# Note that while each script call produces two output files, to be
-# parallel-make safe we need to split this into two rules. (See for
-# example gram.y for more explanation.)
+# parallel-make safe we need to split this into two rules. (See notes
+# in src/backend/parser/Makefile about rules with multiple outputs.)
#
define map_rule
MAPS += $(1)_to_utf8.map utf8_to_$(1).map