Found the following significant warnings:
  readstat/readstat_bits.h:10:29: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
  readstat/readstat_bits.c:11:29: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
  readstat/readstat_parser.c:6:40: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
  readstat/readstat_variable.c:5:41: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
  readstat/readstat_variable.c:8:41: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
  readstat/readstat_writer.c:38:40: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
  readstat/sas/../readstat_bits.h:10:29: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
  readstat/sas/ieee.c:19:15: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
  readstat/sas/ieee.c:99:15: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
  readstat/sas/readstat_sas.c:123:24: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
  readstat/sas/readstat_xport_read.c:48:35: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
  readstat/spss/readstat_por.h:34:24: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
  readstat/spss/readstat_por.c:67:24: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
  readstat/spss/readstat_por_write.c:170:43: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
  readstat/spss/../readstat_bits.h:10:29: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
  readstat/stata/../readstat_bits.h:10:29: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
See '/home/hornik/tmp/R.check/r-devel-clang/Work/PKGS/haven.Rcheck/00install.out' for details.