Prevent incorrect cmake usage

It's a reoccurring issue that users try to build individual components
by pointing cmake at a specific subdirectory, e.g. 'cmake vncviewer'.
CMake, unfortunately, has insufficient protection against this so we'll
need to add a manual check.

This commit only adds it to the most likely places for misuse so we
don't have to pollute every CMakeLists.txt.

tests/CMakeLists.txt

+if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
+  message(FATAL_ERROR "cmake must be invoked with the top level directory")
+endif()
+
 # Benchmarking tools
 add_subdirectory(perf)
 

unix/CMakeLists.txt

+if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
+  message(FATAL_ERROR "cmake must be invoked with the top level directory")
+endif()
+
 add_subdirectory(tx)
 add_subdirectory(common)
 add_subdirectory(vncconfig)

vncviewer/CMakeLists.txt

+if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
+  message(FATAL_ERROR "cmake must be invoked with the top level directory")
+endif()
+
 add_executable(vncviewer
   fltk/Fl_Monitor_Arrangement.cxx
   fltk/Fl_Navigation.cxx

win/CMakeLists.txt

+if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
+  message(FATAL_ERROR "cmake must be invoked with the top level directory")
+endif()
+
 include_directories(${CMAKE_SOURCE_DIR}/common ${CMAKE_SOURCE_DIR}/win)
 
 configure_file(resdefs.h.in ${CMAKE_CURRENT_BINARY_DIR}/resdefs.h)