meta: define _GNU_SOURCE to get strptime() from <time.h>
To use `strptime()`, the documentation indicates
#define _XOPEN_SOURCE
#include <time.h>
However, previously this was done wrongly.
For example, when building with musl we got a warning:
CC meta.lo
meta.c:40: warning: "_XOPEN_SOURCE" redefined
40 | #define _XOPEN_SOURCE
|
In file included from /usr/include/errno.h:8,
from meta.c:13:
/usr/include/features.h:16: note: this is the location of the previous definition
16 | #define _XOPEN_SOURCE 700
|
Defining "__USE_XOPEN" is wrong. This is a glibc internal define not for
the user.
Note that if we just set _XOPEN_SOURCE (or _XOPEN_SOURCE=700), we won't
get other things like "struct tm.tm_gmtoff".
Instead, we already define _GNU_SOURCE at other places. Do that here
too, it will give us strptime() and all is good.
Also, those directives should be defined as first thing (or via "-D"
command line). See [1].
This is also important, because to use "time_t" in a header, we would
need to include <time.h>. That only works, if we get the feature test
macros right. That is, define the _?_SOURCE macro as first thing.