Add caching for cxx_feature_check (#573)

cmake/CXXFeatureCheck.cmake

@@ -27,25 +27,27 @@ function(cxx_feature_check FILE)
     return()
   endif()
 
-  message("-- Performing Test ${FEATURE}")
-  if(CMAKE_CROSSCOMPILING)
-    try_compile(COMPILE_${FEATURE}
-            ${CMAKE_BINARY_DIR} ${CMAKE_CURRENT_SOURCE_DIR}/cmake/${FILE}.cpp
-            CMAKE_FLAGS ${BENCHMARK_CXX_LINKER_FLAGS}
-            LINK_LIBRARIES ${BENCHMARK_CXX_LIBRARIES})
-    if(COMPILE_${FEATURE})
-      message(WARNING
-            "If you see build failures due to cross compilation, try setting HAVE_${VAR} to 0")
-      set(RUN_${FEATURE} 0)
+  if (NOT DEFINED COMPILE_${FEATURE})
+    message("-- Performing Test ${FEATURE}")
+    if(CMAKE_CROSSCOMPILING)
+      try_compile(COMPILE_${FEATURE}
+              ${CMAKE_BINARY_DIR} ${CMAKE_CURRENT_SOURCE_DIR}/cmake/${FILE}.cpp
+              CMAKE_FLAGS ${BENCHMARK_CXX_LINKER_FLAGS}
+              LINK_LIBRARIES ${BENCHMARK_CXX_LIBRARIES})
+      if(COMPILE_${FEATURE})
+        message(WARNING
+              "If you see build failures due to cross compilation, try setting HAVE_${VAR} to 0")
+        set(RUN_${FEATURE} 0)
+      else()
+        set(RUN_${FEATURE} 1)
+      endif()
     else()
-      set(RUN_${FEATURE} 1)
+      message("-- Performing Test ${FEATURE}")
+      try_run(RUN_${FEATURE} COMPILE_${FEATURE}
+              ${CMAKE_BINARY_DIR} ${CMAKE_CURRENT_SOURCE_DIR}/cmake/${FILE}.cpp
+              CMAKE_FLAGS ${BENCHMARK_CXX_LINKER_FLAGS}
+              LINK_LIBRARIES ${BENCHMARK_CXX_LIBRARIES})
     endif()
-  else()
-    message("-- Performing Test ${FEATURE}")
-    try_run(RUN_${FEATURE} COMPILE_${FEATURE}
-            ${CMAKE_BINARY_DIR} ${CMAKE_CURRENT_SOURCE_DIR}/cmake/${FILE}.cpp
-            CMAKE_FLAGS ${BENCHMARK_CXX_LINKER_FLAGS}
-            LINK_LIBRARIES ${BENCHMARK_CXX_LIBRARIES})
   endif()
 
   if(RUN_${FEATURE} EQUAL 0)