Add a fix for compiling mapnik against boost 1.53.0.

I had trouble compiling mapnik on RHEL 6 with boost 1.53.0. This patch fixed my particular problem, and it didn't break the compilation on clang either.

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.

I am willing to refine this patch until it fits your coding style.

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FYI,  I tested with clang:
Apple LLVM version 4.2 (clang-425.0.27) (based on LLVM 3.2svn)
Target: x86_64-apple-darwin12.3.0
Thread model: posix