gpredict does not build with MUSL libc

[dl1jbe]

Compiling under MUSL libc fails with 'implicit-function-declaration' for 'gethostbyname()' in gtk-rig.ctrl.c and gtk-rot-ctrl.c.

Further investigation shows that both files need the following feature macro to be defined:

#ifndef _GNU_SOURCE #define _GNU_SOURCE #endif

Under MUSL netdb.h exports gethostbyname() only if the macro is defined.