diff --git a/BasiliskII/src/Windows/configure.ac b/BasiliskII/src/Windows/configure.ac
index 33540135..bb8e1a22 100755
--- a/BasiliskII/src/Windows/configure.ac
+++ b/BasiliskII/src/Windows/configure.ac
@@ -280,10 +280,13 @@ if [[ "x$HAVE_GCC30" = "xyes" ]]; then
   AC_CACHE_CHECK([whether the compiler supports -fno-strict-aliasing],
     ac_cv_gcc_no_strict_aliasing, [
     AC_TRY_COMPILE([],[],
-      [ac_cv_gcc_no_strict_aliasing=yes; AC_SUBST(SLIRP_CFLAGS, "-fno-strict-aliasing")],
+      [ac_cv_gcc_no_strict_aliasing=yes],
       [ac_cv_gcc_no_strict_aliasing=no])
   ])
   CFLAGS="$SAVED_CFLAGS"
+  if test "x$ac_cv_gcc_no_strict_aliasing" = xyes; then
+       AC_SUBST(SLIRP_CFLAGS, "-fno-strict-aliasing")
+  fi
 fi
 
 dnl Select appropriate CPU source and REGPARAM define.
diff --git a/SheepShaver/src/Windows/configure.ac b/SheepShaver/src/Windows/configure.ac
index 73adb19d..f646bb6e 100644
--- a/SheepShaver/src/Windows/configure.ac
+++ b/SheepShaver/src/Windows/configure.ac
@@ -189,10 +189,13 @@ if [[ "x$HAVE_GCC30" = "xyes" ]]; then
   AC_CACHE_CHECK([whether the compiler supports -fno-strict-aliasing],
     ac_cv_gcc_no_strict_aliasing, [
     AC_TRY_COMPILE([],[],
-      [ac_cv_gcc_no_strict_aliasing=yes; AC_SUBST(SLIRP_CFLAGS, "-fno-strict-aliasing")],
+      [ac_cv_gcc_no_strict_aliasing=yes],
       [ac_cv_gcc_no_strict_aliasing=no])
   ])
   CFLAGS="$SAVED_CFLAGS"
+  if test "x$ac_cv_gcc_no_strict_aliasing" = xyes; then
+       AC_SUBST(SLIRP_CFLAGS, "-fno-strict-aliasing")
+  fi
 fi
 
 case $host_os in