I ran into this building on Solaris 10:

cc -D_GNU_SOURCE -errtags=yes  -O  -DXML_STATIC -DHAVE_LIBXML2_CONFIG_H -DXML_NS
 -o relaxng.o -DHAVE_STDINT_H  -I../../libxml2src -I../../libxml2src/include  -c
 -I../../libxml2src -I../../libxml2src/include  ../../libxml2src/relaxng.c
"../../libxml2src/relaxng.c", line 5372: operands have incompatible types:
         pointer to char ":" pointer to const unsigned char
"../../libxml2src/relaxng.c", line 5372: warning: argument #5 is incompatible wi
th prototype:
        prototype: pointer to const unsigned char : "../../libxml2src/relaxng.c"
, line 510
        argument : pointer to char (E_ARG_INCOMPATIBLE_WITH_ARG)
"../../libxml2src/relaxng.c", line 9462: warning: argument #3 is incompatible wi
th prototype:
        prototype: pointer to const unsigned char : "../../libxml2src/relaxng.c"
, line 2347
        argument : pointer to char (E_ARG_INCOMPATIBLE_WITH_ARG)
cc: acomp failed for ../../libxml2src/relaxng.c


I fixed it as follows:
5372,5373c5370
<                    (node == NULL ? "nothing" : node->name), NULL);
<         return (NULL);
---
>                    (node == NULL ? BAD_CAST "nothing" : node->name), NULL); return (NULL);
9462,9464c9459,9460
<                     VALID_ERR2(XML_RELAXNG_ERR_INTEREXTRA, "noname");
<                 } else {
<                     VALID_ERR2(XML_RELAXNG_ERR_INTEREXTRA, cur->name);
---
>                     VALID_ERR2(XML_RELAXNG_ERR_INTEREXTRA, BAD_CAST "noname");
>                 } else { VALID_ERR2(XML_RELAXNG_ERR_INTEREXTRA, cur->name);