#include <tcl.h>
  ...
  #if ((TCL_MAJOR_VERSION < 8) || ((TCL_MAJOR_VERSION == 8) \
      && (TCL_MINOR_VERSION == 0)))
  extern char * Tcl_PkgPresent _ANSI_ARGS_((Tcl_Interp *, char *, char *, int));
  #endif

  #if ((TCL_MAJOR_VERSION < 8) || ((TCL_MAJOR_VERSION == 8) \
      && (TCL_MINOR_VERSION == 0)))
  char *
  Tcl_PkgPresent(Tcl_Interp* interp, char* name, char* version, int exact)
  {
      /*
         Must use writable buffer to support Tcl 7.
         The Tcl_DStringAppend interface became CONST in Tcl 8
       */
      char *cmd = "package provide";
      Tcl_DString buf;
      Tcl_DString loadedVersion;

      /*
         Use [package provide] to query if the named package is
         already in the interpreter.  Tcl_PkgProvide() doesn't
         return the version, so a Tcl_Eval() is necessary.
       */

      Tcl_DStringInit(&buf);
      Tcl_DStringAppend(&buf, cmd, -1);
      Tcl_Eval(interp, Tcl_DStringAppendElement(&buf, name));
      Tcl_DStringFree(&buf);

      Tcl_DStringInit(&loadedVersion);
      Tcl_DStringGetResult(interp, &loadedVersion);
      if ( !(Tcl_DStringLength(&loadedVersion)) ) {
          Tcl_DStringFree(&loadedVersion);
          if (version != NULL) {
              Tcl_AppendResult(interp, "package ", name, " ", version,
                      " is not present", (char *) NULL);
          } else {
              Tcl_AppendResult(interp, "package ", name,
                      " is not present", (char *) NULL);
          }
          return NULL;
      }
      Tcl_DStringFree(&loadedVersion);

      /*
         At this point we know some version is loaded, so 
         Tcl_PkgRequire will have no side effect.  Call it
         to do the rest of the work.
       */

      return Tcl_PkgRequire(interp, name, version, exact);
  }
  #endif