AP_Math: rename macros to avoid conflicts

these macros were also defined in NuttX in clock.h

libraries/AP_Math/AP_Math.h

@@ -183,32 +183,32 @@ static inline auto MAX(const A &one, const B &two) -> decltype(one > two ? one :
 
 inline uint32_t hz_to_nsec(uint32_t freq)
 {
-    return NSEC_PER_SEC / freq;
+    return AP_NSEC_PER_SEC / freq;
 }
 
 inline uint32_t nsec_to_hz(uint32_t nsec)
 {
-    return NSEC_PER_SEC / nsec;
+    return AP_NSEC_PER_SEC / nsec;
 }
 
 inline uint32_t usec_to_nsec(uint32_t usec)
 {
-    return usec * NSEC_PER_USEC;
+    return usec * AP_NSEC_PER_USEC;
 }
 
 inline uint32_t nsec_to_usec(uint32_t nsec)
 {
-    return nsec / NSEC_PER_USEC;
+    return nsec / AP_NSEC_PER_USEC;
 }
 
 inline uint32_t hz_to_usec(uint32_t freq)
 {
-    return USEC_PER_SEC / freq;
+    return AP_USEC_PER_SEC / freq;
 }
 
 inline uint32_t usec_to_hz(uint32_t usec)
 {
-    return USEC_PER_SEC / usec;
+    return AP_USEC_PER_SEC / usec;
 }
 
 /*

libraries/AP_Math/definitions.h

@@ -64,10 +64,14 @@ static const double WGS84_B = (WGS84_A * (1 - WGS84_F));
 // Eccentricity of the Earth
 static const double WGS84_E = (sqrt(2 * WGS84_F - WGS84_F * WGS84_F));
 
-#define NSEC_PER_SEC    1000000000ULL
-#define NSEC_PER_USEC   1000ULL
-#define USEC_PER_SEC    1000000ULL
-#define USEC_PER_MSEC   1000ULL
-#define MSEC_PER_SEC    1000ULL
-#define SEC_PER_WEEK    (7ULL * 86400ULL)
-#define MSEC_PER_WEEK   (SEC_PER_WEEK * MSEC_PER_SEC)
+/*
+  use AP_ prefix to prevent conflict with OS headers, such as NuttX
+  clock.h
+ */
+#define AP_NSEC_PER_SEC   1000000000ULL
+#define AP_NSEC_PER_USEC  1000ULL
+#define AP_USEC_PER_SEC   1000000ULL
+#define AP_USEC_PER_MSEC  1000ULL
+#define AP_MSEC_PER_SEC   1000ULL
+#define AP_SEC_PER_WEEK   (7ULL * 86400ULL)
+#define AP_MSEC_PER_WEEK  (AP_SEC_PER_WEEK * AP_MSEC_PER_SEC)