INSTALL.md should note libboost min version

[kayD]

I tried to install mapnik 2 on Ubuntu oneiric and installed libboost 1.42 first. There were errors compiling with python scons/scons.py

g++ -o src/mapped_memory_cache.os -c -DHAVE_JPEG -DBOOST_REGEX_HAS_ICU -ansi -Wall -pthread -ftemplate-depth-300 -DLINUX -DBOOST_SPIRIT_THREADSAFE -DMAPNIK_THREADSAFE -O3 -finline-functions -Wno-inline -Wno-parentheses -Wno-char-subscripts -DNDEBUG -DHAVE_CAIRO -fPIC -I/usr/include/cairo -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -I/usr/include/pixman-1 -I/usr/include/libpng12 -I/usr/include/cairomm-1.0 -I/usr/lib/cairomm-1.0/include -I/usr/include/sigc++-2.0 -I/usr/lib/sigc++-2.0/include -Ideps/agg/include -I. -Iinclude -I/usr/local/include -I/usr/include -I/usr/include/freetype2 -I/usr/include/libxml2 -I/usr/include/postgresql src/mapped_memory_cache.cpp
In file included from /usr/include/boost/interprocess/detail/utilities.hpp:29:0,
                 from /usr/include/boost/interprocess/mapped_region.hpp:20,
                 from include/mapnik/mapped_memory_cache.hpp:33,
                 from src/mapped_memory_cache.cpp:26:
/usr/include/boost/interprocess/detail/transform_iterator.hpp:57:15: error: reference ‘m_value’ cannot be declared ‘mutable’ [-fpermissive]
scons: *** [src/mapped_memory_cache.os] Error 1
scons: building terminated because of errors.
make: *** [mapnik] Error 2

I could resolve this problem by removing libboost 1.42 and installing libboost 1.46. So the documentation should
state the required minimum version for libboost, or (better) it should be checked in the .configure script.

[springmeyer]

Boost 1.42 works fine in most cases, but yes I have seen that error on ubuntu. I think it is a boost bug that is likely fixed in some packages of 1.42 (or only affects certain g++ versions). I compile frequently for example, mapnik with boost 1.42 on Mac OS, without problem.

[kayD]

Boost 1.42 works fine in most cases

Ok, maybe it's worth mentioning anyway that arising problems may be resolved by switching to boost 1.46, assuming that upgrading poses no other problems.