BUILD: makefile: fix build options when building tools first

BUILD: makefile: fix build options when building tools first

Due to the test on the target introduced by commit 9577a152b ("BUILD:
makefile: do not erase build options for some build options"), if a
tool (e.g. halog) is build first before haproxy after a clean or a
fresh source extraction, the .build_opts file does not exist and
"make" complains since there's no such target. Make sure to define
the empty target for all "else" blocks there. No backport is needed.