bacula: fix cross compiling for armv7l

got this error while cross compiling:

checking whether setpgrp takes no argument... configure: error: cannot
check setpgrp when cross compiling

autoconf manual says:

— Macro: AC_FUNC_SETPGRP

    If setpgrp takes no argument (the Posix version), define SETPGRP_VOID. Otherwise, it is the BSD version, which takes two process IDs as arguments. This macro does not check whether setpgrp exists at all; if you need to work in that situation, first call AC_CHECK_FUNC for setpgrp.

    The result of this macro is cached in the ac_cv_func_setpgrp_void variable.

    This macro is obsolescent, as current systems have a setpgrp whose signature conforms to Posix. New programs need not use this macro.

So override it with caching variable.