Better Linux support

[letoh]

Since the macro _DEFAULT_SOURCE was introduced in glibc 2.19, build current pfroth will still be failed in the system with older glibc (as far as I know, some Linux systems such as Slackware and CentOS still use glibc 2.17).

According to the description in the manpage:

Since glibc 2.19, defining _GNU_SOURCE also has the effect of
implicitly defining _DEFAULT_SOURCE. In glibc versions before
2.20, defining _GNU_SOURCE also had the effect of implicitly
defining _BSD_SOURCE and _SVID_SOURCE.

and the recommendation from the manual:

We recommend you use _GNU_SOURCE in new programs. If you don’t specify the ‘-ansi’ option to GCC, or other conformance options such as -std=c99, and don’t define any of these macros explicitly, the effect is the same as defining _DEFAULT_SOURCE to 1.

I simply change the feature test macro to _GNU_SOURCE for better Linux support.