Fixup windows build.

Fallout from f01580ce, vs2012 was
complaining about mismatching IE_VER / WINVER in boost.

Change-Id: I2092aba9fa64740eb796538855caaefae9bceab6

external/boost/StaticLibrary_boostthread.mk

@@ -11,6 +11,10 @@ $(eval $(call gb_StaticLibrary_StaticLibrary,boostthread))
 
 $(eval $(call gb_StaticLibrary_set_warnings_not_errors,boostthread))
 
+$(eval $(call gb_StaticLibrary_add_defs,boostthread,\
+       -D_WIN32_WINNT=0x0502 \
+))
+
 # disable "auto link" "feature" on MSVC
 $(eval $(call gb_StaticLibrary_add_defs,boostthread,\
 	-DBOOST_ALL_NO_LIB \