Fixes #105: 1.0.0 clock_gettime runtime failure built with macOS 10.11 and Xcode 8.x

XCode 8 SDK declares clock_gettime() but macOS 10.11 and earlier do not
actually implement it. So just checking for a clock_gettime()
declaration in configure is insufficient.

Instead, do AC_SEARCH_LIBS first to check if -lrt is required, then use
AC_CHECK_FUNCS to check for clock_gettime() availability.

configure.ac

@@ -330,13 +330,6 @@ AC_CHECK_SIZEOF(size_t)
 AC_FUNC_MMAP
 AC_FUNC_STRERROR_R
 
-AC_CHECK_DECL([clock_gettime], 
-    AC_DEFINE([HAVE_CLOCK_GETTIME], 1, [Define if you have clock_gettime() declared in <time.h>]),,
-    [
-#define _XOPEN_SOURCE 500
-#include <time.h>
-    ]
-)
 AC_SEARCH_LIBS([clock_gettime], [rt]) 
 
 save_CFLAGS="$CFLAGS"
@@ -346,6 +339,7 @@ LIBS="$PTHREAD_LIBS $LIBS"
 
 AC_CHECK_FUNCS([ \
 alarm \
+clock_gettime \
 directio \
 fdatasync \
 gettimeofday \