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.

configure

@@ -21006,6 +21006,10 @@ fi
             pthread_cflags="-D_REENTRANT"
             pthread_lib="-L/usr/lib -lpthread"
             ;;
+        *-*-haiku*)
+            pthread_cflags="-D_REENTRANT"
+            pthread_lib=""
+            ;;
         *)
             pthread_cflags="-D_REENTRANT"
             pthread_lib="-lpthread"
@@ -22394,7 +22398,9 @@ fi
             SDL_LIBS="-lcygwin $SDL_LIBS"
         fi
         ;;
-    *-*-beos* | *-*-haiku*)
+
+
+    *-*-haiku*)
         ARCH=beos
         ac_default_prefix=/boot/common
         CheckDummyVideo
@@ -22402,6 +22408,7 @@ fi
         CheckDummyAudio
         CheckBWINDOW
         CheckBeGL
+        CheckPTHREAD
         # Set up files for the audio library
         if test x$enable_audio = xyes; then
 
@@ -22418,16 +22425,6 @@ $as_echo "#define SDL_JOYSTICK_BEOS 1" >>confdefs.h
             SOURCES="$SOURCES $srcdir/src/joystick/beos/*.cc"
             have_joystick=yes
         fi
-        # Set up files for the thread library
-        if test x$enable_threads = xyes; then
-
-$as_echo "#define SDL_THREAD_BEOS 1" >>confdefs.h
-
-            SOURCES="$SOURCES $srcdir/src/thread/beos/*.c"
-            SOURCES="$SOURCES $srcdir/src/thread/generic/SDL_sysmutex.c"
-            SOURCES="$SOURCES $srcdir/src/thread/generic/SDL_syscond.c"
-            have_threads=yes
-        fi
         # Set up files for the timer library
         if test x$enable_timers = xyes; then