gh-126877: Fix the configure check for Tcl/Tk with optimizing compilers (GH-153543)

The check assigned the addresses of Tcl_Init() and Tk_Init() to unused
variables, which optimizing compilers can eliminate, so it linked even
when the libraries were missing.  Call the functions instead.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: serhiy-storchaka

Misc/NEWS.d/next/Build/2026-07-11-07-26-16.gh-issue-126877.Zt7Kq2.rst

@@ -0,0 +1,2 @@
+Fix the :program:`configure` check for Tcl/Tk which could wrongly succeed
+with optimizing compilers when the libraries are missing.

configure

@@ -4618,8 +4618,8 @@ WITH_SAVE_ENV([
       #  error "Tk older than 8.5.12 not supported"
       #endif
     ], [
-      void *x1 = Tcl_Init;
-      void *x2 = Tk_Init;
+      Tcl_Init(NULL);
+      Tk_Init(NULL);
     ])
   ], [
     have_tcltk=yes