configure: Add option --enable-Werror to add -Werror to CFLAGS.

configure: Add option --enable-Werror to add -Werror to CFLAGS.

-Werror is useful for development, but it screws up configure because it's
impossible to guess what new warnings compilers will add in the future.
This commit adds a new configure option to add CFLAGS after the configure
checks are done.

The use of AC_CONFIG_COMMANDS_PRE is based on Eric Blake's suggestion on
the autoconf mailing list: "AC_CONFIG_COMMANDS_PRE probably fits the bill
as the ideal macro to use for guaranteeing that you inject your shell code
at the last possible moment."

The Autoconf code added in this commit, as well as this commit message,
come from Open vSwitch.