Since USD 21.11 the libraries are prefixed with "usd_", i.e. "libusd_m.a" became "libusd_usd_m.a". This commit adjusts our usd.cmake (for building USD) and FindUSD.cmake (for finding the USD libraries) so that they work with and without this usd_ prefix.
Only tested on Linux, not on macOS/Windows.
See for more info: https://github.com/PixarAnimationStudios/USD/blob/release/CHANGELOG.md#2111---2021-11-01