Add x86_64-win64-gcc to supported targets

libvpx builds and runs correctly when built with x86_64-w64-mingw32-gcc
Version tested: 4.6.1

Change-Id: I9aa953d615551ca4834c9a5f8b68778d032a6cf5

configure

diff --git a/configure b/configure
index e6fcc87..cca94a2 100755
--- a/configure
+++ b/configure
@@ -121,6 +121,7 @@
 all_platforms="${all_platforms} x86_64-linux-gcc"
 all_platforms="${all_platforms} x86_64-linux-icc"
 all_platforms="${all_platforms} x86_64-solaris-gcc"
+all_platforms="${all_platforms} x86_64-win64-gcc"
 all_platforms="${all_platforms} x86_64-win64-vs8"
 all_platforms="${all_platforms} x86_64-win64-vs9"
 all_platforms="${all_platforms} universal-darwin8-gcc"