Do not call the XmRegisterConverters function because current Motif
does not declare it (but still exports it).  With this call, the
result of this configure check depends on whether the compiler
supports implicit function declarations or not.

diff -ur grace-5.1.25.orig/ac-tools/aclocal.m4 grace-5.1.25/ac-tools/aclocal.m4
--- grace-5.1.25.orig/ac-tools/aclocal.m4	2014-08-16 23:54:51.000000000 +0200
+++ grace-5.1.25/ac-tools/aclocal.m4	2022-12-22 10:22:57.579378019 +0100
@@ -280,7 +280,6 @@
       int main(void) {
         int vlibn, vincn;
         vincn = XmVersion;
-        XmRegisterConverters();
         vlibn = xmUseVersion;
         if (vincn < [$1]) {
           exit(1);
diff -ur grace-5.1.25.orig/configure grace-5.1.25/configure
--- grace-5.1.25.orig/configure	2022-12-22 10:24:10.869603703 +0100
+++ grace-5.1.25/configure	2022-12-22 10:23:12.843216734 +0100
@@ -9556,7 +9556,6 @@
       int main(void) {
         int vlibn, vincn;
         vincn = XmVersion;
-        XmRegisterConverters();
         vlibn = xmUseVersion;
         if (vincn < 1002) {
           exit(1);