CMake: Fix time.h checks not running on second CMake run.

If CMake was configured more than once, HAVE_CLOCK_GETTIME and
HAVE_CLOCK_MONOTONIC would not be set as compile definitions. The check
for librt being needed to provide HAVE_CLOCK_GETTIME was also
simplified.

CMakeLists.txt

@@ -150,24 +150,26 @@ tuklib_integer(ALL)
 
 # Check for clock_gettime(). Do this before checking for threading so
 # that we know there if CLOCK_MONOTONIC is available.
-if(NOT WIN32 AND NOT DEFINED HAVE_CLOCK_GETTIME)
+if(NOT WIN32)
     check_symbol_exists(clock_gettime time.h HAVE_CLOCK_GETTIME)
+
     if(NOT HAVE_CLOCK_GETTIME)
         # With glibc <= 2.17 or Solaris 10 this needs librt.
-        unset(HAVE_CLOCK_GETTIME CACHE)
-
+        # Add librt for the next check for HAVE_CLOCK_GETTIME. If it is
+        # found after including the library, we know that librt is required.
         list(INSERT CMAKE_REQUIRED_LIBRARIES 0 rt)
-        check_symbol_exists(clock_gettime time.h HAVE_CLOCK_GETTIME)
+        check_symbol_exists(clock_gettime time.h HAVE_CLOCK_GETTIME_LIBRT)
 
-        # If it was found now, add it to all targets and keep it
-        # in CMAKE_REQUIRED_LIBRARIES for further tests too.
-        if(HAVE_CLOCK_GETTIME)
+        # If it was found now, add librt to all targets and keep it in
+        # CMAKE_REQUIRED_LIBRARIES for further tests too.
+        if(HAVE_CLOCK_GETTIME_LIBRT)
             link_libraries(rt)
         else()
             list(REMOVE_AT CMAKE_REQUIRED_LIBRARIES 0)
         endif()
     endif()
-    if(HAVE_CLOCK_GETTIME)
+
+    if(HAVE_CLOCK_GETTIME OR HAVE_CLOCK_GETTIME_LIBRT)
         add_compile_definitions(HAVE_CLOCK_GETTIME)
 
         # Check if CLOCK_MONOTONIC is available for clock_gettime().