BUILD: Auto detect taskbar support.

Formerly the taskbar support was *always* enabled except when the backend or
user specified that it should be disabled. This causes nasty crashes for
backends which do not have any taskbar support (like DC, Tizen and probably
more which simply did not disable it so far) when defaultErrorHandler was
called for example (Mass Add is also broken for those).
The SDL (and derived backends) worked around missing taskbar support by
simply faking a dummy taskbar implementation (but still claiming in configure
that we feature taskbar integration, ouch).

To avoid all non-SDL backends from manually specifying _taskbar=no I added
some auto detection code which simply only enables taskbar support in case
ScummVM is built on Win32 or libunity is present.

configure

@@ -126,7 +126,7 @@ _opengl=auto
 _opengles=auto
 _readline=auto
 _freetype2=auto
-_taskbar=yes
+_taskbar=auto
 _updates=no
 _libunity=auto
 # Default option behavior yes/no
@@ -2427,7 +2427,6 @@ if test -n "$_host"; then
 			CXXFLAGS="$CXXFLAGS -fno-delayed-branch"
 			_backend="dc"
 			_build_scalers=no
-			_taskbar=no
 			_mad=yes
 			_zlib=yes
 			add_line_to_config_mk 'ronindir = /usr/local/ronin'
@@ -2686,7 +2685,6 @@ if test -n "$_host"; then
 			_backend="tizen"
 			_port_mk="backends/platform/tizen/tizen.mk"
 			_arm_asm=yes
-			_taskbar=no
 			_build_scalers=no
 			_seq_midi=no
 			_mt32emu=no
@@ -3989,24 +3987,27 @@ fi
 # Check whether to build taskbar integration support
 #
 echo_n "Building taskbar integration support... "
-define_in_config_if_yes $_taskbar 'USE_TASKBAR'
-if test "$_taskbar" = yes; then
+if test "$_taskbar" = "no"; then
+	echo "no"
+else
 	case $_host_os in
 	mingw*)
 		LIBS="$LIBS -lole32 -luuid"
 		echo "win32"
+		_taskbar=yes
 		;;
 	*)
 		if test "$_libunity" = yes; then
 			echo "unity"
+			_taskbar=yes
 		else
-			echo "$_taskbar"
+			echo "no"
+			_taskbar=no
 		fi
 		;;
 	esac
-else
-	echo "$_taskbar"
 fi
+define_in_config_if_yes $_taskbar 'USE_TASKBAR'
 
 #
 # Check whether to build Bink video support