the last parameter of XChangeProperty is the number of elements.. and when the element format is 32.. the element is "long" so we have 5 long elements here.

Yes this seems confusing as on mac+linux Long is either 32 or 64bits depending on the architecture, but this is how the X11 protocol is defined. Thus 5 is the correct value for the nelts here.  Not 5 or 10 depending on the architecture.

More info on the confusion https://bugs.freedesktop.org/show_bug.cgi?id=16802

docs/README-cmake.md

@@ -0,0 +1,32 @@
+CMake
+================================================================================
+(www.cmake.org)
+
+SDL's build system was traditionally based on autotools. Over time, this
+approach has suffered from several issues across the different supported 
+platforms.
+To solve these problems, a new build system based on CMake is under development.
+It works in parallel to the legacy system, so users can experiment with it
+without complication.
+While still experimental, the build system should be usable on the following
+platforms:
+
+* FreeBSD
+* Linux
+* VS.NET 2010
+* MinGW and Msys
+* OS X with support for XCode
+
+
+================================================================================
+Usage
+================================================================================
+
+Assuming the source for SDL is located at ~/sdl
+
+    cd ~
+    mkdir build
+    cd build
+    cmake ../sdl
+
+This will build the static and dynamic versions of SDL in the ~/build directory.