Add a fix for compiling mapnik against boost 1.53.0.

Qualify the call to phoenix::construct so that the compiler on RHEL6
(gcc (GCC) 4.4.6 20120305 (Red Hat 4.4.6-4)) doesn't complain. The
compiler complains that "boost::phoenix::tag::construct" is not a function.
Merge pull request #1787 from dancasimiro/fix-compile

Add a fix for compiling mapnik against boost 1.53.0.