devel/rapidcheck: fix build on powerpc64 elfv2

-Werror shouldn't be used by default:
/wrkdirs/usr/ports/devel/rapidcheck/work/rapidcheck-d9482c6/include/rapidcheck/gen/Numeric.hpp:46:43: error: implicit conversion from 'std::__1::numeric_limits<unsigned long>::type' (aka 'unsigned long') to 'double' changes value from 18446744073709551615 to 18446744073709551616 [-Werror,-Wimplicit-int-float-conversion]