Compiling 6.4.2: Flex problem

I downloaded latest 6.4.2 and tried compiling on linux 2.0.34 (redhat
5.1) with latest
flex version 2.5.4
But the make complains - you have buggy version of flex!! Help.

make[1]: Entering directory
`/home/src/redhat/BUILD/postgresql-6.4.2/src'
make -C lextest all
make[2]: Entering directory
`/home/src/redhat/BUILD/postgresql-6.4.2/src/lextest'
flex scan.l
gcc -c lex.yy.c
gcc -c lextest.c
gcc -o lextest lex.yy.o lextest.o
make[2]: Leaving directory
`/home/src/redhat/BUILD/postgresql-6.4.2/src/lextest'

You have a buggy version of flex.  Read INSTALL and search for flex for
a fix.

make[1]: *** [lexverify] Error 1
make[1]: Leaving directory `/home/src/redhat/BUILD/postgresql-6.4.2/src'

make: *** [all] Error 2