cmake: improper c std pinning

[mafemergency]

neither gcc nor clang declare the function lrintf in c89 mode while the build script pins this version:
https://github.com/xiph/opusfile/blob/master/CMakeLists.txt#L71
https://c.godbolt.org/z/voebhnGPE

further, a user cannot configure their desired c std as in cmake ... -DCMAKE_C_STANDARD=99 as the target_compile_options command will override this choice

looking at the opus build script i see no mention of std anywhere, and neither does it force-enable warnings that a user can't configure, and i think opusfile should follow that lead