diff --git a/m4/fp_settings.m4 b/m4/fp_settings.m4
index 02fd987efc46996abe8d905655cdc0d4b53cad84..59d01f32d3dfd36bc5001b1f1028c90eae8e6bdb 100644
--- a/m4/fp_settings.m4
+++ b/m4/fp_settings.m4
@@ -44,7 +44,7 @@ dnl ghc-toolchain.
 AC_DEFUN([SUBST_TOOLDIR],
 [
     dnl and Note [How we configure the bundled windows toolchain]
-set -- "$(echo "$$1" | sed 's%'"$mingw_prefix"'%'"$mingw_install_prefix"'%g')"
+    $1=`echo "$$1" | sed 's%'"$mingw_prefix"'%'"$mingw_install_prefix"'%g'`
 ])
 
 # FP_SETTINGS