libvpx

Disable __longjmp_chk protection

glibc implements some checking on longjmp() calls by replacing it with
an internal function __longjmp_chk(), when FORTIFY_SOURCE is defined.
This can be problematic when compiling the library under one version of
glibc and running it under another. Work around this issue for the one
symbol affected for now, before taking out the undef hammer.

Fixes http://code.google.com/p/webm/issues/detail?id=166

Change-Id: Ifc5e25cdec17915e394711f2185b3e9214572d10