Android defines linux, but doesn't have the gettid system call.
--- a/src/thread/pthread/SDL_systhread.c Fri Mar 25 13:47:49 2011 -0700
+++ b/src/thread/pthread/SDL_systhread.c Fri Mar 25 13:48:48 2011 -0700
@@ -23,7 +23,7 @@
#include <pthread.h>
#include <signal.h>
-#ifdef linux
+#ifdef __LINUX__
#include <sys/time.h>
#include <sys/resource.h>
#include <sys/syscall.h>
@@ -100,7 +100,7 @@
int
SDL_SYS_SetThreadPriority(SDL_ThreadPriority priority)
{
-#ifdef linux
+#ifdef __LINUX__
int value;
if (priority == SDL_THREAD_PRIORITY_LOW) {