diff --git a/jtag/ChangeLog b/jtag/ChangeLog
index cdfda448..6f890978 100644
--- a/jtag/ChangeLog
+++ b/jtag/ChangeLog
@@ -3,6 +3,7 @@
     * configure.ac: Improve detection of flex version, get Subversion
           revision from 1. svn info, or 2. ChangeLog, or 3. Revision tag, 
 		  auto-detect whether -lrt is needed
+    * src/svf/svf_flex.l: derive HAVE_MODERN_FLEX internally
     * src/jtag.c: show version from config.h, compile without readline
     * src/version.awk: not needed; SVN info now available in config.h
     * src/Makefile.am: removed -lrt link and version.h target
diff --git a/jtag/configure.ac b/jtag/configure.ac
index ae5aa1b8..54281f2e 100644
--- a/jtag/configure.ac
+++ b/jtag/configure.ac
@@ -112,7 +112,6 @@ if test "$LEX" == flex ; then
    fi
 
    if test "$modern_flex" == yes ; then
-      AC_DEFINE([HAVE_MODERN_FLEX], 1, [Define if flex version is >= $flex_ref_version])
       AC_MSG_RESULT([yes - flex $flex_version])
       AM_CONDITIONAL([BISON_LOCATIONS], true)
    else
diff --git a/jtag/src/svf/svf_flex.l b/jtag/src/svf/svf_flex.l
index 416551ca..e17e376d 100644
--- a/jtag/src/svf/svf_flex.l
+++ b/jtag/src/svf/svf_flex.l
@@ -28,9 +28,10 @@
  * Note: This lexer supports bison_locations if HAVE_MODERN_FLEX is
          defined. Apart from this, the option --bison-locations has
          to be specified on the command line.
-         Both the define and the command line option are controlled
-         by the configure script that enables this feature based on
-         the available version of flex.
+         The command line option is controlled by the configure
+         script that enables this feature based on the available
+         version of flex. The #define is derived below from the version
+         information that flex itself embeds in the *.c output.
  */
 
 
@@ -45,7 +46,17 @@
 static int map_keyw_ident(YYSTYPE *, char *);
 static void align_string(char *);
 
-/* autodetected by configure based on current flex version */
+#ifdef YY_FLEX_MAJOR_VERSION
+ #if (YY_FLEX_MAJOR_VERSION > 2) ||\
+     ((YY_FLEX_MAJOR_VERSION == 2) && ((YY_FLEX_MINOR_VERSION > 5) \
+        || ((YY_FLEX_MINOR_VERSION == 5) && (YY_FLEX_SUBMINOR_VERSION >= 31))))
+  #undef HAVE_MODERN_FLEX
+  #define HAVE_MODERN_FLEX 1
+ #else
+  #undef HAVE_MODERN_FLEX
+ #endif
+#endif
+
 #ifdef HAVE_MODERN_FLEX
 static void fix_yylloc(YYLTYPE *, char *);
 static void fix_yylloc_nl(YYLTYPE *, char *);