gpsutils: Fixes build on OSX.

OSX needs _DARWIN_C_SOURCE defined to get strlcpy() (copied from
driver_ais.c).  There's some inconsistency in the config test,
perhaps related to its being a builtin.

TESTED:
Ran "build-all check" on most of the usual set of machines and VMs,
with no issues.  Only skipped macOS 10.14, due to the large cost
of rebooting into it at this time.

1 files changed, 3 insertions, 0 deletions

diff --git a/gpsutils.c b/gpsutils.c
index 988e4fb7..49a6959d 100644
--- a/gpsutils.c
+++ b/gpsutils.c
@@ -15,6 +15,9 @@
 #define  _POSIX_C_SOURCE 200112L
 #endif
 
+/* strlcpy() needs _DARWIN_C_SOURCE */
+#define _DARWIN_C_SOURCE
+
 #include <ctype.h>
 #include <errno.h>
 #include <math.h>