scons: Tell MSVC STL library to not use exceptions.

MSVC defaults to no exceptions unless /EH option is passed (which we don't), while
MSVC's STL defaults to use exceptions unless _HAS_EXCEPTIONS=0 is defined,
which we didn't.

This fixes

  warning C4530: C++ exception handler used, but unwind semantics are not enabled. Specify /EHsc

Reviewed-by: Brian Paul <brianp@vmware.com>

scons/gallium.py

@@ -351,6 +351,7 @@ def generate(env):
                 '_SCL_SECURE_NO_WARNINGS',
                 '_SCL_SECURE_NO_DEPRECATE',
                 '_ALLOW_KEYWORD_MACROS',
+                '_HAS_EXCEPTIONS=0', # Tell C++ STL to not use exceptions
             ]
         if env['build'] in ('debug', 'checked'):
             cppdefines += ['_DEBUG']