[libc++] Fix GNU/Hurd build

GNU/Hurd does have clock_gettime, it just doesn't define _POSIX_TIMERS because its support for timers is not complete.

Reviewed By: #libc, Mordante

Differential Revision: https://reviews.llvm.org/D158584

(cherry picked from commit 1cfcc36)

Author: sthibaul

libcxx/src/chrono.cpp

@@ -31,7 +31,7 @@
 # include <sys/time.h> // for gettimeofday and timeval
 #endif
 
-#if defined(__APPLE__) || (defined(_POSIX_TIMERS) && _POSIX_TIMERS > 0)
+#if defined(__APPLE__) || defined (__gnu_hurd__) || (defined(_POSIX_TIMERS) && _POSIX_TIMERS > 0)
 # define _LIBCPP_HAS_CLOCK_GETTIME
 #endif
 

libcxx/src/filesystem/filesystem_clock.cpp

@@ -29,7 +29,7 @@
 # include <sys/time.h> // for gettimeofday and timeval
 #endif
 
-#if defined(__APPLE__) || (defined(_POSIX_TIMERS) && _POSIX_TIMERS > 0)
+#if defined(__APPLE__) || defined (__gnu_hurd__) || (defined(_POSIX_TIMERS) && _POSIX_TIMERS > 0)
 # define _LIBCPP_HAS_CLOCK_GETTIME
 #endif