Add a build variable collect_metrics to the scons build.
Add a build variable collect_metrics to the scons build.

Invoking the build with 'scons collect_metrics=1' will
compile the library -DOC_COLLECT_METRICS, which enables
recording training metrics.

svn path=/trunk/theora/; revision=17692

brion/theora,brion/theora,brion/theora,brion/theora,brion/theora