Fix:

tclxml.c:252:7: error: implicit declaration of function 'Tcldom_libxml2_Init' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
  if (Tcldom_libxml2_Init(interp) != TCL_OK) {
      ^

https://cgit.freebsd.org/ports/commit/?id=af36765e17ba36d0e86e95d4476b67f894209468
--- tclxml.c.orig
+++ tclxml.c
@@ -20,6 +20,8 @@
 #include <tclxslt/tclxslt.h>
 #include <string.h>
 
+int Tcldom_libxml2_Init ( Tcl_Interp *interp );
+
 #define TCL_DOES_STUBS \
     (TCL_MAJOR_VERSION > 8 || TCL_MAJOR_VERSION == 8 && (TCL_MINOR_VERSION > 1 || \
     (TCL_MINOR_VERSION == 1 && TCL_RELEASE_LEVEL == TCL_FINAL_RELEASE)))