Started BeOS removal: merged BeOS thread and pthread code.

Haiku uses most of the standard pthread API, with a few #ifdefs where we
still need to fallback onto the old BeOS APIs.

BeOS, however, does not support pthreads (or maybe doesn't support it well),
so I'm unplugging support for the platform with this changeset. Be Inc went
out of business in 2001.

cmake/sdlchecks.cmake

@@ -638,6 +638,9 @@ macro(CheckPTHREAD)
     elseif(HPUX)
       set(PTHREAD_CFLAGS "-D_REENTRANT")
       set(PTHREAD_LDFLAGS "-L/usr/lib -pthread")
+    elseif(BEOS)
+      set(PTHREAD_CFLAGS "-D_REENTRANT")
+      set(PTHREAD_LDFLAGS "")
     else()
       set(PTHREAD_CFLAGS "-D_REENTRANT")
       set(PTHREAD_LDFLAGS "-lpthread")