posix: add feature test macro _GNU_SOURCE for pow10(3) and pow10f(3)
posix: add feature test macro _GNU_SOURCE for pow10(3) and pow10f(3)

https://bugzilla.gnome.org/show_bug.cgi?id=614788

GNOME/vala,frida/vala,GNOME/vala,GNOME/vala,frida/vala,frida/vala,frida/vala,frida/vala,GNOME/vala