Rename Cmake variables for clarity

example/advanced/server-flex/CMakeLists.txt

@@ -13,7 +13,7 @@ if (OPENSSL_FOUND)
     GroupSources(example/advanced/server-flex "/")
 
     add_executable (advanced-server-flex
-        ${BOOST_BEAST_INCLUDES}
+        ${BOOST_BEAST_FILES}
         ${PROJECT_SOURCE_DIR}/example/common/detect_ssl.hpp
         ${PROJECT_SOURCE_DIR}/example/common/server_certificate.hpp
         ${PROJECT_SOURCE_DIR}/example/common/ssl_stream.hpp