Don't ever attempt to initialise a std::locale with a UTF-8 locale on Windows

As I said in 2ff121f2, passing a
POSIX-style locale name to the std::locale constructor on Windows is
decidedly odd, and apparently boost adds insult to injury by even
really attempting to use a UTF-8 one if that is what our code thinks
it wants. Or something.

This commit avoids the ton of
"warn:unotools.i18n:7996:3544:unotools/source/i18n/resmgr.cxx:123: CRT
Report Hook: ASSERT: f:\dd\vctools\crt\crtw32\stdcpp\xmbtowc.c(89) :
Assertion failed: ploc->_Mbcurmax == 1 || ploc->_Mbcurmax == 2" we
otherwise get when built with --enable-dbgutil and a current VS2015
(or 2017?).

Change-Id: I3a774dff7b12d9960f553c9f242e0463c6d13d5d
Reviewed-on: https://gerrit.libreoffice.org/54444Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tor Lillqvist <tml@collabora.com>

external/boost/UnpackedTarball_boost.mk

@@ -36,6 +36,8 @@ boost_patches += boost_1_63_0.undef.warning.patch.1
 # https://svn.boost.org/trac10/ticket/13230
 boost_patches += boost-android-unified.patch.1
 
+boost_patches += windows-no-utf8-locales.patch.0
+
 $(eval $(call gb_UnpackedTarball_UnpackedTarball,boost))
 
 $(eval $(call gb_UnpackedTarball_set_tarball,boost,$(BOOST_TARBALL)))

external/boost/windows-no-utf8-locales.patch.0

+Don't ever attempt to initialise a std::locale with a UTF-8 locale on Windows -*- Mode: Diff -*-
+
+--- libs/locale/src/std/std_backend.cpp
++++ libs/locale/src/std/std_backend.cpp
+@@ -109,12 +109,13 @@
+                 #endif
+             }
+             else {
++                #if !defined(BOOST_WINDOWS)
+                 if(loadable(lid)) {
+                     name_ = lid;
+                     utf_mode_ = utf8_native_with_wide;
+                 }
+-                #if defined(BOOST_WINDOWS)
+-                else if(loadable(win_name)) {
++                #else
++                if(loadable(win_name)) {
+                     name_ = win_name;
+                     utf_mode_ = utf8_from_wide;
+                 }