scons: add /dynamicbase and /nxcompat to MSVC linkflags

This builds the opengl DLLs with address layout space randomization
(ASLR) and data execution prevention (DEP) for better security.

Reviewed-by: Kurt Daverman <krd@vmware.com>

scons/gallium.py

@@ -547,6 +547,8 @@ def generate(env):
         linkflags += [
             '/fixed:no',
             '/incremental:no',
+            '/dynamicbase',
+            '/nxcompat',
         ]
     env.Append(LINKFLAGS = linkflags)
     env.Append(SHLINKFLAGS = shlinkflags)