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)
fix showing mp_connect between sp scenarios

show_connect ("configure sides" in advanced settings dialog) is now carried over between scenarios, and is used to determine whether to show mp_connect between scenarios. it is used as the default value of allow_new_game_flag, so setting allow_new_game in config will override show_connect for scenarios after the first one. also, mp_connect will be shown between scenarios regardless of settings if debug mode is enabled (this is unchanged).
fix 1.13.0-dev comability

games from older 1.13.0-dev versions weren't loaded correctly because comparision operators for version_info cannot handle -dev versions.
fix for sp campaign commandline handling

reworked to select scenario in configure_engine (one place for sp/mp).
also, the old code had several errors which have been fixed.
Proper implementation for clear_overlay_area.
SDL_gpu implementation for removing floating labels.
Fixed wrong speaker
Merge branch 'master' of https://github.com/wesnoth/wesnoth
Added secondary unit filters to sighted events

Added secondary unit filters to all sighted events I found which seemed
like they might suffer from the same problem as bug #22466.

Fixes bug #22466.
wml tools GUI: fixed the WinError 87 that happened on Python 3.4
wml tools GUI: improved credits description for the Tango icons
wml tools GUI: replaced all IntVar() instances used by checkbuttons with BooleanVar()
wml tools GUI: removed the 0 from some IntVar() initializations, because it doesn't do anything
MAI test scenario: fix string that caused trouble for translations

Somehow the combination of “” and new lines caused the string to be
terminated prematurely when it was extracted into the .pot file.  Fixed
by removing the “”.
only add multiplayer info for sp campaign saves

before, configure_engine was being run on all saves including unit tests, which caused an assertion failure.
Dragon MAI test scenario: turn off debug output

The messages spoken by the hunter (dragon) are debug output and not
translatable.  They should not appear in the scenario.
Merge pull request #268 from techtonik/master

scons: improve dependencies detection on Windows (MinGW)