Experimental pthread compatibility library

This patch adds pthread compatibility by using libmthread.

To use this with a program using pthreads, you have to replace
  #include <pthread>
with
  #define _MTHREADIFY_PTHREADS
  #include <minix/mthreads>

This also changes the initialization function to be a constructor, which
is implicitly called before the call to main. This allows for
conformance with pthreads, while not paying a high price by checking on
each mthread_* call whether the library has been initialized or not.

As mthread_init is now a constructor, it also has been set as static, and
relevent calls removed from programs using it.

Change-Id: I2aa375db557958d2bee9a70d285aabb990c88f00

lib/Makefile

@@ -19,11 +19,14 @@ SUBDIR+=	../external/gpl3/gcc/lib/libgcc .WAIT
 . endif
 .endif
 
-#LSC MINIX libc depends on libminlib because of minix/malloc-debug.c
+#LSC MINIX libc depends on
+# - libminlib because of minix/malloc-debug.c
+# - libmthread because of sys/lib/libunwind
 SUBDIR+=	libminlib
 SUBDIR+=	.WAIT
 
 SUBDIR+=	libsys
+SUBDIR+=	libmthread
 SUBDIR+=	.WAIT
 
 SUBDIR+=	libc
@@ -43,7 +46,6 @@ SUBDIR+=	libasyn \
 		libinputdriver \
 		libminc \
 		libminixfs \
-		libmthread \
 		libnetdriver \
 		libsffs \
 		libtimers \