pthreadw32: module.defs fixes.

Remove libpthread.a symlink during UNINSTALL; its our
responsibility since we create it at the INSTALL stage.

Don't add build targets to both BUILD.extra and BUILD.ntargets;
since both fields are added to BUILD.args, this was resulting in
duplicate targets being passed to make.

Move (real)clean target from BUILD to CLEAN, as this is where it
belongs; note that CLEAN is automatically called by the build
system as part as the BUILD stage. Previously, parallel builds
could call the (real)clean target after GC-static, resulting in
a build failure during the INSTALL stage.

contrib/pthreadw32/module.defs

@@ -7,8 +7,10 @@ PTHREADW32.FETCH.md5 = 36ba827d6aa0fa9f9ae740a35626e2e3
 
 PTHREADW32.CONFIGURE = $(TOUCH.exe) $@
 
-PTHREADW32.BUILD.extra = CROSS=$(BUILD.cross.prefix) clean GC-static
-PTHREADW32.BUILD.ntargets = clean GC-static
+PTHREADW32.CLEAN.ntargets = realclean
+
+PTHREADW32.BUILD.extra = CROSS=$(BUILD.cross.prefix)
+PTHREADW32.BUILD.ntargets = GC-static
 
 define PTHREADW32.INSTALL
     $(CP.exe) $(PTHREADW32.EXTRACT.dir/)pthread.h $(CONTRIB.build/)include/
@@ -27,6 +29,7 @@ endef
 
 define PTHREADW32.UNINSTALL
     $(RM.exe) -f $(CONTRIB.build/)lib/libpthreadGC2.a
+    $(RM.exe) -f $(CONTRIB.build/)lib/libpthread.a
     $(RM.exe) -f $(CONTRIB.build/)include/pthread.h
     $(RM.exe) -f $(CONTRIB.build/)include/sched.h
     $(RM.exe) -f $(CONTRIB.build/)include/semaphore.h