scons: Promote pointer arithmetic to error.

All code covered by scons is being built on multiple OSes, so pointer
arithmetic must really be addressed when spotted.

scons/gallium.py

@@ -366,7 +366,7 @@ def generate(env):
         ccflags += [
             '-Wall',
             '-Wmissing-field-initializers',
-            '-Wpointer-arith',
+            '-Werror=pointer-arith',
             '-Wno-long-long',
             '-ffast-math',
             '-fmessage-length=0', # be nice to Eclipse