Apply COPT to CXXFLAGS as well
The main use of that make variable is to pass in -Werror. It makes
sense to apply this to C++ as well.
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/flat/
fe3e200c-edee-44e0-a6e3-
d45dca72873b%40eisentraut.org