= prjconf =
Macros:
%with_MYVAR %%nosuchvar

= spec =
%bcond_with MYVAR

%prep
echo "with_MYVAR:%{with_MYVAR}"
%if %{with MYVAR}
echo "with_MYVAR is set!"
%else
echo "with_MYVAR is *not* set!"
%endif
exit 1

= osc build =
[    6s] + echo with_MYVAR:%nosuchvar
[    6s] with_MYVAR:%nosuchvar
[    6s] + echo 'with_MYVAR is set!'
[    6s] with_MYVAR is set!
[    6s] + exit 1