Index: apps/build.tcl
===================================================================
diff -u -N -r5a80e290968c4a48ef91cf714f522edb8a75c6d5 -r0aae129621a59088bba8470cd7aef9ce2221c4b7
--- apps/build.tcl	(.../build.tcl)	(revision 5a80e290968c4a48ef91cf714f522edb8a75c6d5)
+++ apps/build.tcl	(.../build.tcl)	(revision 0aae129621a59088bba8470cd7aef9ce2221c4b7)
@@ -81,11 +81,6 @@
   switch -exact -- $TOOLCHAIN {
     autoconf-tea {
       set opts [list --libdir=$tclDir --enable-64bit]
-
-      # see https://core.tcl-lang.org/tips/doc/trunk/tip/538.md
-      if {$TCLTAG eq "core-8-branch"} {
-        lappend opts "--without-system-libtommath"
-      }
       
       exec >@stdout 2>@stderr bash -lc "./configure $opts"
       exec >@stdout 2>@stderr bash -lc "make"
Index: generic/nsf.c
===================================================================
diff -u -N -r6ef097d5e5cc5515e5aa2960cf2132a5aa258391 -r0aae129621a59088bba8470cd7aef9ce2221c4b7
--- generic/nsf.c	(.../nsf.c)	(revision 6ef097d5e5cc5515e5aa2960cf2132a5aa258391)
+++ generic/nsf.c	(.../nsf.c)	(revision 0aae129621a59088bba8470cd7aef9ce2221c4b7)
@@ -16652,7 +16652,23 @@
  *----------------------------------------------------------------------
  */
 
+#if TCL_MAJOR_VERSION > 8 || TCL_MINOR_VERSION > 6
+/*
+ * Starting with Tcl 8.7a4 and TIP 538, NSF might end up built against Tcl
+ * linking against a system-wide/ external libtommath, rather than with an
+ * embedded libtommath. In both cases, even the embedded one, Tcl does not
+ * ship tommat.h anymore. This leaves NSF without the necessary build-time
+ * definitions for mp_int and mp_clear (see below). For the time being, we
+ * rely on a hot fix by the TIP 538 author, providing compat definitions when
+ * setting the TCL_NO_TOMMATH_H macro before including tclTomMath.h.
+ *
+ * See https://core.tcl-lang.org/tcl/tktview?name=4663e0636f (also for other
+ * mid-term options)
+ */
+#define TCL_NO_TOMMATH_H 1
+#endif
 #include <tclTomMath.h>
+
 int Nsf_ConvertToInteger(Tcl_Interp *interp, Tcl_Obj *objPtr,  const Nsf_Param *pPtr,
                      ClientData *clientData, Tcl_Obj **outObjPtr)
   nonnull(1) nonnull(2) nonnull(3) nonnull(4) nonnull(5);