--- a/configure.ac
+++ b/configure.ac
@@ -3,10 +3,10 @@
 AC_CONFIG_SRCDIR([lib/Xm/Form.c])
 AC_PREREQ(2.52)
 AC_CONFIG_AUX_DIR(.)
-AC_CHECK_FILE(/usr/X/include/X11/X.h,
+AS_IF([test -f "/usr/X/include/X11/X.h"],
   AC_PREFIX_DEFAULT(/usr/X),
   AC_PREFIX_DEFAULT(/usr))
-AC_CHECK_FILE(/usr/X11R6/include/X11/X.h,
+AS_IF([test -f "/usr/X11R6/include/X11/X.h"],
   AC_PREFIX_DEFAULT(/usr/X11R6),
   AC_PREFIX_DEFAULT(/usr))
 
