Minor doc update - use original "ungrouped" build option from SDL.

There is a plan to group all the build options from SDL into its own group, however, it has not been implemented yet in the master branch. These paragrahps were cherry-picked from a refactoring branch so the information there were more forward-looking.

Docs/GettingStarted.dox

@@ -13,12 +13,12 @@ Although all required third-party libraries are included as source code, there a
 - For Windows, the June 2010 DirectX SDK needs to be installed. This is not necessary if building on Visual Studio 2012 or newer, which install the Windows SDK with the necessary DirectX files.
 
 - For Linux, install one or more of the development packages from each software component category below. Building as 32-bit on a 64-bit system requires installing also the 32-bit versions of the development libraries.
-  + %Database server (optional). Disabled by default even when the development package is installed, use URHO3D_DATABASE_ODBC build option to enable. The two packages below are mutually exclusive. The ODBC driver manager is the recommended package.
+  + %Database server (optional). Disabled by default even when the development package is installed, use URHO3D_DATABASE_ODBC=1 to enable. The two packages below are mutually exclusive. The ODBC driver manager is the recommended package.
     - libiodbc2-dev (Debian-based) or libiodbc-devel (RedHat-based) for iODBC driver manager.
     - unixodbc-dev (Debian-based) or unixODBC-devel (RedHat-based) for ODBC driver manager.
   + Display server (essential). One or more of these can be installed at the same time. When multiple display servers are available, X11 takes precedence (overridable using SDL_VIDEODRIVER environment variable during application runtime).
-    - libdirectfb-dev (Debian-based only) for Direct Frame Buffer (DirectFB). Disabled by default even when installed, use SDL_OPT_VIDEO_DIRECTFB build option to enable.
-    - libegl1-mesa-dev and mir-client-platform-mesa-dev (Ubuntu only) for Mir.
+    - libdirectfb-dev (Debian-based only) for Direct Frame Buffer (DirectFB). Disabled by default even when installed, use VIDEO_DIRECTFB=1 to enable.
+    - libegl1-mesa-dev and mir-client-platform-mesa-dev (Ubuntu only) for Mir. Only Mir version 0.24.0 or above is supported. Disabled by default even when installed, use VIDEO_MIR=1 to enable.
     - libegl1-mesa-dev, libwayland-dev, and wayland-protocols (Debian-based) or libwayland-client-devel, libwayland-cursor-devel, mesa-libwayland-egl-devel, and wayland-protocols-devel (RedHat-based) for Wayland.
     - libx11-dev, libxcursor-dev, libxext-dev, libxi-dev, libxinerama-dev, libxrandr-dev, libxrender-dev, libxss-dev, and libxxf86vm-dev (Debian-based) or libX11-devel, libXcursor-devel, libXext-devel, libXi-devel, libXinerama-devel, libXrandr-devel, libXrender-devel, libXScrnSaver-devel, and libXxf86vm-devel (RedHat-based) for X11 and its extensions. Alternatively, you can use xorg-devel meta package on Debian-based system, but it would install other unneeded packages as well.
   + %Sound server (essential). One or more of these can be installed at the same time. When multiple sound servers are available, PulseAudio takes precedence (overridable using SDL_AUDIODRIVER environment variable during application runtime).
@@ -26,7 +26,7 @@ Although all required third-party libraries are included as source code, there a
     - arts-devel (RedHat-based only) for Analog Real %Time Synthesizer (aRts).
     - libaudio-dev (Debian-based only) for %Network %Audio System (NAS).
     - libesd0-dev (Debian-based) or esound-devel (RedHat-based) for Enlightened %Sound Daemon (ESounD).
-    - libfusionsound-dev (Debian-based only, not available on Ubuntu) for FusionSound. Disabled by default even when installed, use SDL_OPT_FUSIONSOUND build option to enable.
+    - libfusionsound-dev (Debian-based only, not available on Ubuntu) for FusionSound. Disabled by default even when installed, use FUSIONSOUND=1 to enable.
     - libpulse-dev (Debian-based) or pulseaudio-libs-devel (RedHat-based) for PulseAudio.
     - libroar-dev (Debian-based only) for RoarAudio (SNDIO).
   + %Input method editor (optional). One or both of these can be installed at the same time to enable the IME support in the game engine. When both frameworks are installed on user's host system, the application uses XMODIFIFIERS environment variable to determine which one is active.