Define `_GNU_SOURCE` properly/consistently per autoconf
Although `_GNU_SOURCE` can be defined as an arbitrary #define per the
glibc docs [1], it's best to define it in a manner consistent with the way
that autoconf defines it, i.e., `1`.
While this shouldn't matter in most cases, it does when the headers from
other projects follow the poorly defined GNU convention implemented by
autoconf and are included after the libevent's util.h header. An example
failure with clang, similar to the failure I encountered, is as follows:
```
$ printf "#define _GNU_SOURCE\n#define _GNU_SOURCE 1" | clang -c -x c -
<stdin>:2:9: warning: '_GNU_SOURCE' macro redefined [-Wmacro-redefined]
^
<stdin>:1:9: note: previous definition is here
^
1 warning generated.
```
This happened when compiling python [2] with a stale homebrew util.h file from
libevent (which admittedly would not happen in a correct libevent install, as the
header should be installed under /usr/local/include/event2/util.h). However, if
both headers had been combined (which is more likely), it would have failed as
shown above.
Removing the ad hoc definition unbreaks compiling python's pyconfig.h.in header
when included after util.h from libevent.
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