added support for a shuffle key in  the [music] music to allow selecting between random and non-random music play
added modifications to changelog
shuffle sides based on the settings value

Before, it was based on the preference value. This was a problem when not using the mp_configure dialog (sp)
Merge pull request #265 from Zappaman/music_shuffle_feature_2

add option to not shuffle music, see https://gna.org/bugs/?19653
SConstruct: Use full condition evaluation when checking dependencies
scons: make have_sdl_* check flags functions that evaluate lazily on demand

This moves messages about optional SDL libs checks below
potential warning about missing base dependencies.
scons: check GCC compiler before SDL
scons: Show info when base prerequisites are met
scons: find bzip2 statically compiled into Boost (windows)