configure.ac: Fix quoting on AC_CHECK_HEADERS

Currently the output from autoheader contains the definition
HAVE_SYS_EVENTFD_H_ (not the trailing underscore) so the sys/eventfd.h
header is never used. Remove a layer of quoting which seems to fix
the issue.
src/common/knote.c: Portably print unitptr_t values

The format string for uintptr_t values is defined in inttypes.h
which is part of C99. Use that instead of having to special case
every 64bit architecture.