compile errors in config phase with ArchLinux and Fedora 20

config.log trying to link with -I/usr/lib/librt.so:
/usr/lib/librt.so: undefined reference to `gettimeofday@GLIBC_2.2.5'

or Fedora20:
/usr/lib/librt.so: error adding symbols: File in wrong format

linkage is AC_LIB_HAVE_LINKFLAGS macro behaviour
see http://marc.info/?l=gnulib-bug&m=129660262901148

Signed-off-by: Dirk Neukirchen <dirkneukirchen@web.de>