Fixes bug #2074 - Thanks Sylvain!

SDL_syssem.c:159 comparison of unsigned expression >= 0 is always true
Solved by comparing unsigneds directly

SDL_systimer.c:164: warning: control may reach end of Compile
Solved by returning the default value if all else fails.

SDL_androidgl.c:41:1: warning: type specifier missing, defaults to 'int'
SDL_androidgl.c:47:1: warning: control reaches end of non-void function
Solved by adding void return type to the function implementation