Adding a configure flag to control WebM container support

Adding a --(enable|disable)-webm-io flag to control WebM container input and
output support. For now, enabling WebM IO by default only when there is a C++
compiler. Doing so because eventually we will move WebM IO to libwebm and it
is built using C++.

Change-Id: I210ac36c23528e382ed41d3c4322291720481492
4 files changed