This extra _DEFAULT_SOURCE definition results in a double definition
which is a compile error.
This fixes the following compile error with glibc:
----------------------------------------------------------------------
ugps-2019-06-25-cd7eabcd/nmea.c:19: error: "_DEFAULT_SOURCE" redefined [-Werror]
#define _DEFAULT_SOURCE
<command-line>: note: this is the location of the previous definition
cc1: all warnings being treated as errors
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
8aab70081b