force shell interpretation of compilation for ucontext detection

It appears that $Cofnig{cc} expects that it will be interpreted through a shell, or at least split on whitespace before being sent to system(). This patch uses the former approach. For an example of a failed detection, see http://www.cpantesters.org/cpan/report/df2bb3ca-1009-11e3-a716-c172c209780a