pg_buildext: Copy CFLAGS from environment to COPT
pg_buildext: Copy CFLAGS from environment to COPT so Makefile.global picks it up. Notably this will make extension builds use -f{debug,file}-prefix-map from dpkg-buildflags.
pg_buildext: Copy CFLAGS from environment to COPT so Makefile.global picks it up. Notably this will make extension builds use -f{debug,file}-prefix-map from dpkg-buildflags.