rename KIVY_SDL_PATH TO KIVY_SDL2_PATH and clean up env var query

Author: dessant

doc/sources/guide/environment.rst

@@ -44,7 +44,7 @@ KIVY_HOME
 
     .. versionadded:: 1.9.0
 
-KIVY_SDL_PATH
+KIVY_SDL2_PATH
     If set, the SDL2 libraries and headers from this path are used when
     compiling kivy instead of the ones installed system-wide.
     To use the same libraries while running a kivy app, this path must be

setup.py

@@ -394,10 +394,7 @@ def determine_sdl2():
     if not c_options['use_sdl2']:
         return flags
 
-    if 'KIVY_SDL_PATH' in environ:
-        sdl2_path = environ['KIVY_SDL_PATH'] 
-    else:
-        sdl2_path = None
+    sdl2_path = environ.get('KIVY_SDL2_PATH', None)
 
     flags['libraries'] = ['SDL2', 'SDL2_ttf', 'SDL2_image', 'SDL2_mixer']
     flags['include_dirs'] = ([sdl2_path] if sdl2_path else