scons: Define _USE_MATH_DEFINES on MSVC.
diff --git a/scons/gallium.py b/scons/gallium.py
index 34877b2..a6d69fd 100644
--- a/scons/gallium.py
+++ b/scons/gallium.py
@@ -263,6 +263,7 @@
         if msvc and env['toolchain'] != 'winddk':
             cppdefines += [
                 'VC_EXTRALEAN',
+                '_USE_MATH_DEFINES',
                 '_CRT_SECURE_NO_WARNINGS',
                 '_CRT_SECURE_NO_DEPRECATE',
                 '_SCL_SECURE_NO_WARNINGS',