Avoids troublesome dependency on M_LN2 from math.h.

The M_LN2 definition isn't always provided by math.h, depending on the
settings of the pesky configure flags.  Since ln(2) is a mathematical
constant, there's no loss of generality in defining our own version,
and that decouples us from the variability.

Since the stated purpose of defining _XOPEN_SOURCE in driver_tsip.c
and driver_zodiac.c is just to make M_LN2 available, this is
no longer necessary.

TESTED:
(With subsequent fixes included) Ran "build-all check" on Mac Pro
10.9, Mac Pro 10.14, MacBook Pro 10.9, PowerBook 10.5, VMs for OSX
10.5-10.13, Ubuntu 14.04, CentOS 7, Fedora 25, FreeBSD 10.3, OpenBSD
5.6, OpenBSD 5.6 32-bit, NetBSD 6.1.5, and Beaglebone (arm) Debian 7.

1 files changed, 4 insertions, 0 deletions

diff --git a/gps.h b/gps.h
index 35404615..23a97594 100644
--- a/gps.h
+++ b/gps.h
@@ -2240,6 +2240,10 @@ extern double wgs84_separation(double, double);
 #define RAD_2_DEG	57.2957795130823208767981548141051703
 #define DEG_2_RAD	0.0174532925199432957692369076848861271
 
+/* other mathematical constants */
+#define GPS_LN2         0.693147180559945309417232121458176568
+
+
 /* geodetic constants */
 #define WGS84A 6378137		/* equatorial radius */
 #define WGS84F 298.257223563	/* flattening */