Add EXTRA_CFLAGS to configure

Hi,

I rather regularly want to pass extra flags to configure without
overwriting CFLAGS. There's two basic reasons:

1) ./configure CFLAGS=... essentially breaks --enable-debug and related  options, overwrites -O2 as the default and
such.That's imo pretty  confusing.
 
2) I like to be able to pass -Werror or something to configure without  breaking a bunch of configure tests that won't
workwith Werror.
 

A good way to do that seems to be to add a separate variable for that
purpose? Unless someobdy has a better idea?

Greetings,

Andres Freund