Removed _POSIX_C_SOURCE defines where _GNU_SOURCE was also defined, as _GNU_SOURCE implicitly defines _POSIX_C_SOURCE

[kkysen]

Fixes #220.

As explained in #220, on macOS/BSD, #define _POSIX_C_SOURCE supersedes #define _GNU_SOURCE and prevents the GNU extension asprintf from being defined, which causes a compile error in json2cbor.c. To fix this, we can simply remove the #define _POSIX_C_SOURCE 200809L, as #define _GNU_SOURCE implies it, but without the superseding behavior seen on macOS/BSD.

Specifically, according to https://man7.org/linux/man-pages/man7/feature_test_macros.7.html,

_GNU_SOURCE
Defining this macro (with any value) implicitly defines _ATFILE_SOURCE, _LARGEFILE64_SOURCE, _ISOC99_SOURCE, _XOPEN_SOURCE_EXTENDED, _POSIX_SOURCE, _POSIX_C_SOURCE with the value 200809L (200112L in glibc versions before 2.10; 199506L in glibc versions before 2.5; 199309L in glibc versions before 2.1) and _XOPEN_SOURCE with the value 700 (600 in glibc versions before 2.10; 500 in glibc versions before 2.2). In addition, various GNU-specific extensions are also exposed.

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.

[thiagomacieira]

Makes sense, thanks!