Bad Boost.Context not correctly skipped by the m4 script

[AdamMajer]

Compiling with older boost version where the Context library is not available on some architectures, the boost.m4 fails to skip the detection when the Context library is not present. The cause is a regression in the upstream boost.m4 script. I've submitted it upstream already, but it would be nice if this script was synced for next versions of PowerDNS

tsuna/boost.m4#128

Thanks!

[omoerbeek]

We stopped supporting too old version of boost.