More autoconf housekeeping
Don't let AC_SEARCH_LIBS([lrintf]) add -lm to LIBS, otherwise we'll unconditionally link everything with it. Correctly handle the third possibility of AC_SEARCH_LIBS, that no library at all was found. Link libopusfile with $lrintf_lib, it uses it and will otherwise fail with linkers that use --no-add-needed / --no-copy-dt-needed-entries. Don't bother to test for doxygen if using it is --disable'd.