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.