Makefile: Let "make check-help" work without running ./configure

Currently if you do "make check-help" in a fresh checkout, only an error
is printed which is not nice:

    $ make check-help V=1
    cc -nostdlib  -o check-help.mo
    cc: fatal error: no input files
    compilation terminated.
    rules.mak:115: recipe for target 'check-help.mo' failed
    make: *** [check-help.mo] Error 1

Move the config-host.mak condition into the body of
tests/Makefile.include and always include the rule for check-help.

Reported-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <20170810085025.14076-1-famz@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Fam Zheng <famz@redhat.com>
