Specify that csmith is a C++14 program.

This commit updates the CMake-based build path only.

This commit leaves CXX_STANDARD_REQUIRED unset, which allows the C++
language standard requirement to "decay" as described in the CMake
documentation:
https://cmake.org/cmake/help/latest/prop_tgt/CXX_STANDARD.html

Allowing for "decay" is maybe a bad idea, but I'm not sure, so let it
decay for now.

src/CMakeLists.txt

@@ -1,6 +1,6 @@
 ## -*- mode: CMake -*-
 ##
-## Copyright (c) 2017 The University of Utah
+## Copyright (c) 2017, 2019 The University of Utah
 ## All rights reserved.
 ##
 ## This file is part of `csmith', a random generator of C programs.
@@ -208,6 +208,13 @@ add_executable(csmith
   ${csmith_SOURCES}
   )
 
+# Use C++14.
+set_target_properties(csmith PROPERTIES
+  CXX_STANDARD 14
+  CXX_EXTENSIONS NO
+  )
+# CXX_STANDARD_REQUIRED YES
+
 if(BSD_LIBRARY)
   target_link_libraries(csmith "${BSD_LIBRARY}")
 endif()