Build bootloader: wscript: When using MSVC set minimum target OS to XP.

Set the minimum version of Windows this application will run on.
This is required because since Visual Studio 2012 the default
minimal target was set to Vista (win ver. 6.0).
5.01 is the version number for 32-bit XP, 5.02 for 64-bit XP.

Basically this makes the linker use the option /SUBSYSTEM, with
its four combinations:

/SUBSYSTEM:WINDOWS,5.01
/SUBSYSTEM:CONSOLE,5.01
/SUBSYSTEM:WINDOWS,5.02
/SUBSYSTEM:CONSOLE,5.02

bootloader/wscript

@@ -619,6 +619,10 @@ def configure(ctx):
             # Use Unicode entry point wmain/wWinMain and wchar_t WinAPI
             ctx.env.append_value('CFLAGS', '-DUNICODE')
             ctx.env.append_value('CFLAGS', '-D_UNICODE')
+            # set XP target as minimal target OS ver. when using Windows w/MSVC
+            # https://blogs.msdn.microsoft.com/vcblog/2012/10/08/windows-xp-targeting-with-c-in-visual-studio-2012/
+            ctx.env.append_value('LINKFLAGS', '/SUBSYSTEM:CONSOLE,%s' % (
+                '5.01' if ctx.env.PYI_ARCH == '32bit' else '5.02'))
         else:
             # Use Visual C++ compatible alignment
             ctx.env.append_value('CFLAGS', '-mms-bitfields')
@@ -645,11 +649,19 @@ def configure(ctx):
         ctx.setenv(name, baseenv)  # Inherit from `baseenv`.
         ctx.env.append_value('DEFINES', 'WINDOWED')
 
-        if ctx.env.DEST_OS == 'win32' and ctx.env.CC_NAME != 'msvc':
-            # For MinGW disable console window on Windows - MinGW option
-            # TODO Is it necessary to have -mwindows for C and LINK flags?
-            ctx.env.append_value('LINKFLAGS', '-mwindows')
-            ctx.env.append_value('CFLAGS', '-mwindows')
+        if ctx.env.DEST_OS == 'win32':
+            if ctx.env.CC_NAME != 'msvc':
+                # For MinGW disable console window on Windows - MinGW option
+                # TODO Is it necessary to have -mwindows for C and LINK flags?
+                ctx.env.append_value('LINKFLAGS', '-mwindows')
+                ctx.env.append_value('CFLAGS', '-mwindows')
+            else:
+                _link_flags = ctx.env._get_list_value_for_modification('LINKFLAGS')
+                _subsystem = [x for x in _link_flags if x.startswith('/SUBSYSTEM:')]
+                for parameter in _subsystem:
+                    _link_flags.remove(parameter)
+                ctx.env.append_value('LINKFLAGS', '/SUBSYSTEM:WINDOWS,%s' % (
+                    '5.01' if ctx.env.PYI_ARCH == '32bit' else '5.02'))
         elif ctx.env.DEST_OS == 'darwin':
             # To support catching AppleEvents and running as ordinary OSX GUI
             # app, we have to link against the Carbon framework.