[boost] [1.37][mpl] Merge gcc 4.4 fix.

Hi, It it okay to merge the mpl part of this change to release? http://svn.boost.org/trac/boost/changeset/48960/ It fixes a preprocessor error in gcc 4.4 which stops a lot of libraries from working with that compiler. The python part of the patch isn't required, the error is due to mpl's preprocessed headers not including the boost preprocessor headers and doesn't occur in other libraries that use iteration, as they always include the headers. I've had a look through the test results and everything seems to be working. Daniel