warning: implicit declaration of function ‘asprintf’;

[iam-TJ]

On Ubuntu 18.04 compiling reports:

pgdbf.c: In function ‘main’:
pgdbf.c:571:16: warning: implicit declaration of function ‘asprintf’; did you mean ‘vsprintf’? [-Wimplicit-function-declaration]
             if(asprintf(&pgfields[fieldnum].formatstring, "%%.%dlf", fields[fieldnum].decimals) < 0) {
                ^~~~~~~~
                vsprintf

The man-page details the need to set a feature test macro:

       #define _GNU_SOURCE         /* See feature_test_macros(7) */
       #include <stdio.h>