docs: Add a hint to CMake error message if the required symbol was not found in libBlocksRuntime

Author: ngrewe

CMakeLists.txt

@@ -249,7 +249,7 @@ else ()
 		if (HAVE_BLOCK_USE_RR2)
 			add_definitions(-DHAVE_BLOCK_USE_RR2)
 		else ()
-			message(FATAL_ERROR "libBlocksRuntime does not contain _Block_use_RR2(). Enable EMBEDDED_BLOCKS_RUNTIME to use the built-in blocks runtime.")
+			message(FATAL_ERROR "_Block_use_RR2() could not be found in libBlocksRuntime. Did you install libdispatch with INSTALL_PRIVATE_HEADERS=ON? Alternatively, enable EMBEDDED_BLOCKS_RUNTIME to use the built-in blocks runtime.")
 		endif ()
 		unset(CMAKE_REQUIRED_LIBRARIES)
 	else ()