Require C++11 or higher to compile KWSys

In standalone builds, default to C++11.  Provide options for
host projects to control the standard level.

CMakeLists.txt

@@ -42,6 +42,10 @@
 #      SET(KWSYS_HEADER_ROOT ${PROJECT_BINARY_DIR})
 #      INCLUDE_DIRECTORIES(${PROJECT_BINARY_DIR})
 #
+#  KWSYS_CXX_STANDARD         = A value for CMAKE_CXX_STANDARD within KWSys.
+#                               Set to empty string to use no default value.
+#  KWSYS_CXX_COMPILE_FEATURES = target_compile_features arguments for KWSys.
+#
 # Optional settings to setup install rules are as follows:
 #
 #  KWSYS_INSTALL_BIN_DIR     = The installation target directories into
@@ -86,6 +90,7 @@ CMAKE_MINIMUM_REQUIRED(VERSION 3.1 FATAL_ERROR)
 FOREACH(p
     CMP0056 # CMake 3.2, Honor link flags in try_compile() source-file signature.
     CMP0063 # CMake 3.3, Honor visibility properties for all target types.
+    CMP0067 # CMake 3.8, Honor language standard in try_compile source-file signature.
     CMP0069 # CMake 3.9, INTERPROCEDURAL_OPTIMIZATION is enforced when enabled.
     )
   IF(POLICY ${p})
@@ -112,6 +117,12 @@ SET_PROPERTY(DIRECTORY
   "KWSYS_HEADER(%)=<${KWSYS_NAMESPACE}/%>"
   )
 
+if(KWSYS_CXX_STANDARD)
+  set(CMAKE_CXX_STANDARD "${KWSYS_CXX_STANDARD}")
+elseif(NOT DEFINED CMAKE_CXX_STANDARD AND NOT DEFINED KWSYS_CXX_STANDARD)
+  set(CMAKE_CXX_STANDARD 11)
+endif()
+
 # Select library components.
 IF(KWSYS_STANDALONE OR CMake_SOURCE_DIR)
   SET(KWSYS_ENABLE_C 1)
@@ -875,6 +886,8 @@ IF(KWSYS_C_SRCS OR KWSYS_CXX_SRCS)
       ${KWSYS_TARGET_INTERFACE})
     TARGET_SOURCES(${KWSYS_TARGET_LINK} INTERFACE
       $<TARGET_OBJECTS:${KWSYS_TARGET_OBJECT}>)
+    target_compile_features(${KWSYS_TARGET_OBJECT} PRIVATE ${KWSYS_CXX_COMPILE_FEATURES})
+    target_compile_features(${KWSYS_TARGET_INTERFACE} INTERFACE ${KWSYS_CXX_COMPILE_FEATURES})
   ELSE()
     SET(KWSYS_TARGET_INTERFACE ${KWSYS_NAMESPACE})
     SET(KWSYS_TARGET_OBJECT ${KWSYS_NAMESPACE})
@@ -883,6 +896,7 @@ IF(KWSYS_C_SRCS OR KWSYS_CXX_SRCS)
     SET(KWSYS_LINK_DEPENDENCY PUBLIC)
     ADD_LIBRARY(${KWSYS_TARGET_INTERFACE} ${KWSYS_LIBRARY_TYPE}
       ${KWSYS_C_SRCS} ${KWSYS_CXX_SRCS})
+    target_compile_features(${KWSYS_TARGET_INTERFACE} PUBLIC ${KWSYS_CXX_COMPILE_FEATURES})
   ENDIF()
   if (KWSYS_ALIAS_TARGET)
     add_library(${KWSYS_ALIAS_TARGET} ALIAS ${KWSYS_TARGET_INTERFACE})

kwsysPlatformTests.cmake

@@ -7,11 +7,16 @@ SET(KWSYS_PLATFORM_TEST_FILE_CXX kwsysPlatformTestsCXX.cxx)
 MACRO(KWSYS_PLATFORM_TEST lang var description invert)
   IF(NOT DEFINED ${var}_COMPILED)
     MESSAGE(STATUS "${description}")
+    set(maybe_cxx_standard "")
+    if(CMAKE_VERSION VERSION_LESS 3.8 AND CMAKE_CXX_STANDARD)
+      set(maybe_cxx_standard "-DCMAKE_CXX_STANDARD=${CMAKE_CXX_STANDARD}")
+    endif()
     TRY_COMPILE(${var}_COMPILED
       ${CMAKE_CURRENT_BINARY_DIR}
       ${CMAKE_CURRENT_SOURCE_DIR}/${KWSYS_PLATFORM_TEST_FILE_${lang}}
       COMPILE_DEFINITIONS -DTEST_${var} ${KWSYS_PLATFORM_TEST_DEFINES} ${KWSYS_PLATFORM_TEST_EXTRA_FLAGS}
       CMAKE_FLAGS "-DLINK_LIBRARIES:STRING=${KWSYS_PLATFORM_TEST_LINK_LIBRARIES}"
+                  ${maybe_cxx_standard}
       OUTPUT_VARIABLE OUTPUT)
     IF(${var}_COMPILED)
       FILE(APPEND