cannot build hpcgap on FreeBSD 11.1 due to ERROR: Ward failed to parse C source

[dimpase]

On GAP master (commit ee54ef5e54f626b42a407afeef73d4c29e7d73a3), FreeBSD 11.1 with clang 4.0.0, I get

...
   WARD    src/macfloat.c => gen/macfloat.c
src/macfloat.c:383:38: Syntax error
  return _Generic((VAL_MACFLOAT(f)), float: __signbitf(VAL_MACFLOAT(f)), double: __signbit(VAL_MACFLOAT(f)), long double: __signbitl(VAL_MACFLOAT(f)), volatile float: __signbitf(VAL_MACFLOAT(f)), volatile double: __signbit(VAL_MACFLOAT(f)), volatile long double: __signbitl(VAL_MACFLOAT(f)), volatile const float: __signbitf(VAL_MACFLOAT(f)), volatile const double: __signbit(VAL_MACFLOAT(f)), volatile const long double: __signbitl(VAL_MACFLOAT(f)), const float: __signbitf(VAL_MACFLOAT(f)), const double: __signbit(VAL_MACFLOAT(f)), const long double: __signbitl(VAL_MACFLOAT(f))) ? True : False;
                                     ^
ERROR: Ward failed to parse C source
gmake: *** [Makefile.rules:255: gen/macfloat.c] Error 1

[dimpase]

If instead of clang I use gcc6, I get

   WARD    src/macfloat.c => gen/macfloat.c
src/macfloat.c:383:9: Syntax error
        int
        ^
ERROR: Ward failed to parse C source
gmake: *** [Makefile.rules:255: gen/macfloat.c] Error 1

Note that I can build and run "classic" GAP on this system, so it's definitely Ward/HPCGAP-related.