Make boostthread windows only for now.

Not sure if unix needs it since most folks build with system boost
there.

Change-Id: I49d02a682e6658f6e3c5a6ada480903567d8c436

RepositoryExternal.mk

@@ -326,8 +326,13 @@ else # !SYSTEM_BOOST
 
 $(eval $(call gb_Helper_register_static_libraries,PLAINLIBS, \
 	boostdatetime \
+))
+
+ifeq ($(OS),WNT)
+$(eval $(call gb_Helper_register_static_libraries,PLAINLIBS, \
 	boostthread \
 ))
+endif
 
 define gb_LinkTarget__use_boostdatetime
 $(call gb_LinkTarget_use_static_libraries,$(1),\

boost/Module_boost.mk

@@ -13,7 +13,15 @@ ifeq ($(SYSTEM_BOOST),NO)
 
 $(eval $(call gb_Module_add_targets,boost,\
 	StaticLibrary_boostdatetime \
+))
+
+ifeq ($(OS),WNT)
+$(eval $(call gb_Module_add_targets,boost,\
 	StaticLibrary_boostthread \
+))
+endif
+
+$(eval $(call gb_Module_add_targets,boost,\
 	UnpackedTarball_boost \
 ))
 
@@ -24,6 +32,6 @@ $(eval $(call gb_Module_add_targets,boost,\
 ))
 endif
 
-endif
+endif # SYSTEM_BOOST
 
 # vim: set noet sw=4 ts=4: