diff --git a/configure.in b/configure.in
index 4d14fcac3..5def57326 100644
--- a/configure.in
+++ b/configure.in
@@ -11,6 +11,12 @@ AC_CONFIG_HEADER(tgt-vhdl/vhdl_config.h)
 AC_CANONICAL_HOST
 dnl Checks for programs.
 AC_PROG_CC
+# AC_PROG_CC_C99 is only available in autoconf version 2.60 and later.
+# If you must use an older version then comment out the following two
+# lines, but be warned that there could be issues with finding the
+# nan(), etc. functions. It is really best to upgrade to a supported
+# version of autoconf.
+AC_PREREQ([2.60])
 AC_PROG_CC_C99
 AC_PROG_CXX
 AC_PROG_RANLIB