boost149: MPI fixes.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Formula/boost149.rb

@@ -20,6 +20,8 @@ class Boost149 < Formula
 
   keg_only "Boost 1.49 is provided for software that doesn't compile against newer versions."
 
+  env :userpaths
+
   option :universal
   option 'with-mpi', 'Enable MPI support'
   option 'without-python', 'Build without Python'
@@ -27,6 +29,7 @@ class Boost149 < Formula
 
   depends_on UniversalPython.new if build.universal? and not build.include? "without-python"
   depends_on "icu4c" if build.include? "with-icu"
+  depends_on MPIDependency.new(:cc, :cxx) if build.include? "with-mpi"
 
   fails_with :llvm do
     build 2335