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

doc/src/sgml/installation.sgml

@@ -1937,7 +1937,7 @@ build-postgresql:
      it will break many of <filename>configure</filename>'s built-in tests.  To add
      such flags, include them in the <envar>COPT</envar> environment variable
      while running <filename>make</filename>.  The contents of <envar>COPT</envar>
-     are added to both the <envar>CFLAGS</envar> and <envar>LDFLAGS</envar>
+     are added to the <envar>CFLAGS</envar>, <envar>CXXFLAGS</envar>, and <envar>LDFLAGS</envar>
      options set up by <filename>configure</filename>.  For example, you could do
 <screen>
 <userinput>make COPT='-Werror'</userinput>

src/Makefile.global.in

@@ -741,6 +741,7 @@ endif
 #
 ifdef COPT
    CFLAGS += $(COPT)
+   CXXFLAGS += $(COPT)
    LDFLAGS += $(COPT)
 endif