Separate build directory

When I configure and make in a separate build directory tree, I get

createdb.c:15: dumputils.h: No such file or directory
and
print.c:9: common.h: No such file or directory

in src/bin/scripts. I don't know whether the attached change to the
Makefile is the preferred way to fix this, at least it works for me.

BTW, make creates these files in the source tree:
./src/backend/bootstrap/bootparse.c
./src/backend/bootstrap/bootscanner.c
./src/backend/bootstrap/bootstrap_tokens.h
./src/backend/parser/gram.c
./src/backend/parser/parse.h
./src/backend/parser/scan.c
./src/backend/utils/misc/guc-file.c
./src/bin/psql/sql_help.h
./src/interfaces/ecpg/preproc/pgc.c
./src/interfaces/ecpg/preproc/preproc.c
./src/interfaces/ecpg/preproc/preproc.h
./src/pl/plpgsql/src/pl.tab.h
./src/pl/plpgsql/src/pl_gram.c
./src/pl/plpgsql/src/pl_scan.c

Shouldn't they better be created in the build tree?

Servus
Manfred