Use BOOST_USE_WINDOWS_H to avoid errors when building with clang-cl

> [build CXX] workdir/UnpackedTarball/boost/libs/locale/src/shared/date_time.cpp
[...]
> In file included from C:/lo64/core/workdir/UnpackedTarball/boost/libs/locale/src/shared/date_time.cpp:11:
> In file included from C:/lo64/core/workdir/UnpackedTarball/boost\boost/thread/mutex.hpp:14:
> In file included from C:/lo64/core/workdir/UnpackedTarball/boost\boost/thread/win32/mutex.hpp:9:
> In file included from C:/lo64/core/workdir/UnpackedTarball/boost\boost/thread/win32/basic_timed_mutex.hpp:14:
> C:/lo64/core/workdir/UnpackedTarball/boost\boost/thread/win32/thread_primitives.hpp(172,55):  error: conflicting types for 'CreateMutexA'
>                 __declspec(dllimport) void* __stdcall CreateMutexA(_SECURITY_ATTRIBUTES*,int,char const*);
>                                                       ^
> C:/PROGRA~2/WI3CF2~1/10/Include/10.0.15063.0/um\synchapi.h(489,1):  note: previous declaration is here
> CreateMutexA(
> ^
[...]

Change-Id: I141ea196250dc855c780f237436c0f19f0594cb1

external/boost/StaticLibrary_boost_locale.mk

@@ -15,7 +15,7 @@ $(eval $(call gb_StaticLibrary_set_warnings_not_errors,boost_locale))
 
 # disable "auto link" "feature" on MSVC
 $(eval $(call gb_StaticLibrary_add_defs,boost_locale,\
-	-DBOOST_ALL_NO_LIB -DBOOST_LOCALE_NO_WINAPI_BACKEND -DBOOST_LOCALE_NO_POSIX_BACKEND \
+	-DBOOST_ALL_NO_LIB -DBOOST_LOCALE_NO_WINAPI_BACKEND -DBOOST_LOCALE_NO_POSIX_BACKEND -DBOOST_USE_WINDOWS_H \
 ))
 
 $(eval $(call gb_StaticLibrary_use_external,boost_locale,boost_headers))