diff --git a/tool/buildtclext.tcl b/tool/buildtclext.tcl
index 1118357..0403bd2 100644
--- a/tool/buildtclext.tcl
+++ b/tool/buildtclext.tcl
@@ -32,7 +32,7 @@ set CC {}
 set OPTS {}
 set DESTDIR ""    ; # --destdir "$(DESTDIR)"
 set tclConfigSh ""; # --tclConfig.sh FILE
-set LIBS {}       ; # --extlibs "-lx -ly ..."
+set LIBS {-lm}       ; # --extlibs "-lx -ly ..."
 for {set ii 0} {$ii<[llength $argv]} {incr ii} {
   set a0 [lindex $argv $ii]
   if {$a0=="--install-only"} {