Correctly check for C++17 and higher when declaring timespec_get()

Summary:
In rS338751, the check to declare `timespec_get()` for C++17 and higher
was incorrectly done against a `cplusplus` define, while it should have
been `__cplusplus`.

Fix this by using `__cplusplus`, and also bump `__FreeBSD_version` so it
becomes possible to correctly check for `timespec_get()` in upstream
libc++ headers.

Reviewed by:	brooks, emaste
MFC after:	3 days
Differential Revision: https://reviews.freebsd.org/D22735

include/time.h

@@ -208,7 +208,7 @@ time_t posix2time(time_t t);
 #endif
 
 #if defined(__BSD_VISIBLE) || __ISO_C_VISIBLE >= 2011 || \
-    (defined(cplusplus) && cplusplus >= 201703)
+    (defined(__cplusplus) && __cplusplus >= 201703)
 #include <sys/_timespec.h>
 /* ISO/IEC 9899:201x 7.27.2.5 The timespec_get function */
 #define TIME_UTC	1	/* time elapsed since epoch */

sys/sys/param.h

@@ -60,7 +60,7 @@
  *		in the range 5 to 9.
  */
 #undef __FreeBSD_version
-#define __FreeBSD_version 1300063	/* Master, propagated to newvers */
+#define __FreeBSD_version 1300064	/* Master, propagated to newvers */
 
 /*
  * __FreeBSD_kernel__ indicates that this system uses the kernel of FreeBSD,