Prevent re-execution of slow feature tests.

cmake/CXXFeatureCheck.cmake

@@ -21,12 +21,15 @@ function(cxx_feature_check FILE)
   string(TOLOWER ${FILE} FILE)
   string(TOUPPER ${FILE} VAR)
   string(TOUPPER "HAVE_${VAR}" FEATURE)
+  if (DEFINED HAVE_${VAR})
+    return()
+  endif()
   message("-- Performing Test ${FEATURE}")
   try_run(RUN_${FEATURE} COMPILE_${FEATURE}
           ${CMAKE_BINARY_DIR} ${CMAKE_CURRENT_SOURCE_DIR}/cmake/${FILE}.cpp)
   if(RUN_${FEATURE} EQUAL 0)
     message("-- Performing Test ${FEATURE} -- success")
-    set(HAVE_${VAR} 1 PARENT_SCOPE)
+    set(HAVE_${VAR} 1 CACHE INTERNAL "Feature test for ${FILE}" PARENT_SCOPE)
     add_definitions(-DHAVE_${VAR})
   else()
     if(NOT COMPILE_${FEATURE})