scons: use full condition evaluation when checking dependencies
scons: make SDL availability flags functions that evaluate lazily on demand

This moves messages about optional SDL libs checks below
potential warning about missing base dependencies and
makes SCons check GCC compiler before SDL.
scons: show OK status when base prerequisites are met
scons: find bzip2 statically compiled into Boost (windows)
Merge pull request #268 from techtonik/master

scons: improve dependencies detection on Windows (MinGW)