Downgrade to SDL 2.0.10 until we target macOS >= 10.10

* SDL broke macOS < 10.10 support here: https://hg.libsdl.org/SDL/rev/25c5f567de44
* Even Apple has it wrong (vvailability vs. examples): https://developer.apple.com/documentation/foundation/nsoperatingsystemversion
* Regression in 1340b7b9

build.sh

@@ -25,7 +25,7 @@
 
 FREETYPE_VERSION=2.10.4
 LIBXML_VERSION=2.9.10
-SDL_VERSION=2.0.12
+SDL_VERSION=2.0.10
 TAG_GFXAPPS="current_gfx_apps"
 
 ROOT=`pwd`