cmake: clear some configure checks when the deployment target changes

If the target is changed since the last configure, some checks need to
be looked up again. Clear their cache entries when these checks need to
be redone.

CMakeLists.txt

@@ -98,6 +98,16 @@ foreach(p
   endif()
 endforeach()
 
+# Some configure checks depend upon the deployment target. Clear checks when
+# the deployment target changes.
+if (APPLE)
+  if (NOT CMAKE_OSX_DEPLOYMENT_TARGET STREQUAL KWSYS_LAST_OSX_DEPLOYMENT_TARGET)
+    unset(KWSYS_CXX_HAS_UTIMENSAT CACHE)
+  endif ()
+  set(KWSYS_LAST_OSX_DEPLOYMENT_TARGET "${CMAKE_OSX_DEPLOYMENT_TARGET}"
+    CACHE INTERNAL "remember the last deployment target to trigger configure rechecks")
+endif ()
+
 #-----------------------------------------------------------------------------
 # If a namespace is not specified, use "kwsys" and enable testing.
 # This should be the case only when kwsys is not included inside