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

[rillian]

I think this is a bug in the cmake build description. It looks like cmake sets OP_HAVE_LRINTF if it finds the symbol without setting any of the preprocessor extension selectors which would make the declaration available, like the autotools build does.

OTOH, I don't see this warning building with cmake on fedora linux or openindiana, so maybe there's some other portability issues shadowing?

In any case, building with -std=c99 is fine. We just support c89 for the sake of old embedded toolchains, and using it as the base configuration helps with that.