Log Message:
Update flex to 2.6.4.

Upstream news:

* Noteworthy changes in release 2.6.4 (2017-05-06) [stable]

** build

*** The indent target now knows about flex's new (as of 2.6.0)
    layout. The indent rules it would apply are not correct and do
    need to be fixed.

*** The files included in the flex distribution are now built by the
    version of flex that is included in the distribution.

*** The configure script has a better idea of which headers are
    required to build flex. It will also error when missing functions
    are detected.

*** We have lowered the versions of automake and gettext that
    configure.ac lists as required for building flex. In autogen.sh,
    we now check for how to call libtoolize and use what we find in
    the rest of the script.

*** Since files in lib/ are picked up as needed by src/, we no longer
    generate a Makefile for that directory.

*** Flex can be cross compiled.

** documentation

*** Some typos were removed from the manual.

** scanner

*** Some minor performance enhancements.

*** We honor user defined yy_* macros again. We are also more careful
    to not leak macro definitions into header files.

*** A number of portability fixes were introduced so building flex is
    more reliable on more platforms. Additionally, outdated function
    calls were removed.

*** When building the flex executable itself, %# comments from
    flex.skl are removed when generating the C source code array. This
    reduces the size of flex.

** test suite

*** All scripts in the test suite are now run by $(SHELL) and the
    needed portability fixes have been included.

*** Test suite dependencies are handled much better. This only matters
    if you are actively developing flex or its test suite.

*** Tests that depend on platform dependent features now properly skip
    when those platforms are not present.

*** When running "make check", you can now pas V=0 to silence more of
    the build. This is useful when you're less connncerned about the
    details of building and linking the test programs themselves.