Allow compile with clang 3.8 with warning (#1021)

* Fix missing includes

* clean

* Allow compile with clang 3.8 with warning

* Add message

CMakeLists.txt

@@ -9,9 +9,12 @@ if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
         message (FATAL_ERROR "GCC version must be at least 6! For example, if GCC 6 is available under gcc-6, g++-6 names, do the following: export CC=gcc-6 CXX=g++-6; rm -rf CMakeCache.txt CMakeFiles; and re run cmake or ./release.")
     endif ()
 elseif (CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
-    # Require at least clang 4
+    # Require at least clang 3.8
+    if (CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.8)
+        message (FATAL_ERROR "Clang version must be at least 3.8! Recommended 4+")
+    endif ()
     if (CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4)
-        message (FATAL_ERROR "Clang version must be at least 4!")
+        message (WARNING "Compilation has only been tested with Clang 4+")
     endif ()
 else ()
     message (WARNING "You are using an unsupported compiler! Compilation has only been tested with Clang 4+ and GCC 6+.")