Compile warning: implicit declaration of function ‘asprintf’

[lleaff]

$ cc --version
cc (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ cc -o inplace inplace.c
inplace.c: In function ‘main’:
inplace.c:206:9: warning: implicit declaration of function ‘asprintf’ [-Wimplicit-function-declaration]
     if (asprintf(&tmp_fname, "%s-XXXXXX", fname) == -1)
         ^

After a bit of googling it seems that on some systems _GNU_SOURCE needs to be defined.
Compiling with -D _GNU_SOURCE or adding the following declaration at the top of the file fixes this.:

#ifndef _GNU_SOURCE
#define _GNU_SOURCE 1
#endif