4.16.0 link failure on macOS

[Hanspeter Niederstrasser]

On macOS, building libtasn1-4.16.0 fails with this error:

********
libtool: link: gcc -dynamiclib  -o .libs/libtasn1.6.dylib  .libs/ASN1.o 
.libs/coding.o .libs/decoding.o .libs/element.o .libs/errors.o 
.libs/gstr.o .libs/parser_aux.o .libs/structure.o .libs/version.o 
-Wl,-force_load,gl/.libs/libgnu.a  -L/sw/lib    -install_name 
/sw/lib/libtasn1.6.dylib -compatibility_version 13 -current_version 13.0 
-Wl,-single_module -Wl,-exported_symbols_list,.libs/libtasn1-symbols.expsym
Undefined symbols for architecture x86_64:
  "_c_isdigit", referenced from:
      __asn1_expand_object_id in parser_aux.o
      __asn1_check_identifier in parser_aux.o
********

And looking at the object files, c_isdigit is being used differently in 
parser_aux.c compared to the other places in the code:
****
$ nm -o *.o | grep c_isdigit
ASN1.o: 00000000000041e0 t _c_isdigit
decoding.o: 0000000000005740 t _c_isdigit
element.o: 00000000000014e0 t _c_isdigit
parser_aux.o:                  U _c_isdigit
****

libtasn1-4.15.0 built fine on the same system.

Not a programmer, but happy to test patches.

Hanspeter