server-mode: Add option to enable/disable the mode explicitly

Provide a way for scripts building CMake to enable server mode
explicitly and assume the risk of a build failure if it is not
supported.  This will allow such scripts to ensure that server
mode is available if the build succeeds.  It also allows scripts
to explicitly disable server mode even if it would be supported.

CMakeLists.txt

@@ -695,15 +695,25 @@ endif()
 CMAKE_SETUP_TESTING()
 
 # Check whether to build server mode or not:
-set(CMake_ENABLE_SERVER_MODE 0)
-if(NOT CMake_TEST_EXTERNAL_CMAKE AND CMAKE_USE_LIBUV)
-  list(FIND CMAKE_CXX_COMPILE_FEATURES cxx_auto_type CMake_HAVE_CXX_AUTO_TYPE)
-  list(FIND CMAKE_CXX_COMPILE_FEATURES cxx_range_for CMake_HAVE_CXX_RANGE_FOR)
-  if(CMake_HAVE_CXX_AUTO_TYPE AND CMake_HAVE_CXX_RANGE_FOR)
-    if(CMake_HAVE_CXX_MAKE_UNIQUE)
+if(NOT CMake_TEST_EXTERNAL_CMAKE)
+  if(NOT DEFINED CMake_ENABLE_SERVER_MODE)
+    list(FIND CMAKE_CXX_COMPILE_FEATURES cxx_auto_type CMake_HAVE_CXX_AUTO_TYPE)
+    list(FIND CMAKE_CXX_COMPILE_FEATURES cxx_range_for CMake_HAVE_CXX_RANGE_FOR)
+    if(CMAKE_USE_LIBUV
+        AND CMake_HAVE_CXX_AUTO_TYPE
+        AND CMake_HAVE_CXX_MAKE_UNIQUE
+        AND CMake_HAVE_CXX_RANGE_FOR
+        )
       set(CMake_ENABLE_SERVER_MODE 1)
+    else()
+      set(CMake_ENABLE_SERVER_MODE 0)
     endif()
   endif()
+  if(CMake_ENABLE_SERVER_MODE AND NOT CMAKE_USE_LIBUV)
+    message(FATAL_ERROR "The server mode requires libuv!")
+  endif()
+else()
+  set(CMake_ENABLE_SERVER_MODE 0)
 endif()
 
 if(NOT CMake_TEST_EXTERNAL_CMAKE)