[PATCH nft] configure: Show flex version check

[Shivani Bhardwaj <shivanib134@gmail.com>]

Before this patch, no check on the version of flex was done because of
which configure script did not show any warning messages for
inappropriate version of flex. Following iptables, show the version of
flex in the output of configure script, also show warning if necessary.

Signed-off-by: Shivani Bhardwaj <shivanib134@gmail.com>
---
 configure.ac | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/configure.ac b/configure.ac
index 0e7edcf..b3b6c0b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -50,6 +50,21 @@ then
         exit 1
 fi
 
+AC_MSG_CHECKING(flex version)
+        flex_version=`$ac_cv_prog_LEX --version | sed 's/version//g' | awk '/flex/ {print $2}'`
+        flex_major=`echo $flex_version| cut -d . -f 1`
+        flex_minor=`echo $flex_version| cut -d . -f 2`
+        flex_rev=`echo $flex_version| cut -d . -f 3`
+
+        if (test "$flex_major" -eq "2" && test "$flex_minor" -eq "5" && test "$flex_rev" -ge "33") || (test "$flex_major" -eq "2" && test "$flex_minor" -gt "5") || test "$flex_major" -gt "2"; then
+                AC_MSG_RESULT([$flex_version. OK])
+        else
+                AC_MSG_WARN([flex version $flex_version found.
+                Version 2.5.33 or greater is required. You may experience problems
+                while compilating the nftables compatibility layer for iptables.
+                Please, consider to upgrade flex.])
+        fi
+
 AC_CHECK_PROG(DOCBOOK2X_MAN, [docbook2x-man], [docbook2x-man], [no])
 AC_CHECK_PROG(DOCBOOK2MAN, [docbook2man], [docbook2man], [no])
 AC_CHECK_PROG(DB2X_DOCBOOK2MAN, [db2x_docbook2man], [db2x_docbook2man], [no])
-- 
2.7.4