Make it possible to complie 64-bit binary on windows

The change is to supply `x64` to MSVC_TARGETS if `-8` is specified.
Note that it will set DEST_CPU to `amd64` instead of expected `x86_64`
on Windows. This is known to break things for xash3d-fwgs repo. But for
this one it seems just fine? I haven't investigated it further.

wscript

@@ -87,7 +87,7 @@ def configure(conf):
 	# subsystem=bld.env.MSVC_SUBSYSTEM
 	# TODO: wrapper around bld.stlib, bld.shlib and so on?
 	conf.env.MSVC_SUBSYSTEM = 'WINDOWS,5.01'
-	conf.env.MSVC_TARGETS = ['x86'] # explicitly request x86 target for MSVC
+	conf.env.MSVC_TARGETS = ['x86' if not conf.options.ALLOW64 else 'x64']
 	if sys.platform == 'win32':
 		conf.load('msvc msdev')
 	conf.load('xcompile compiler_c compiler_cxx strip_on_install')