Fix the fix for std::result_of (#4496)

Newer compilers, such as Apple Clang 15.0, have removed `std::result_of`
as part of C++20. The build instructions provided a fix for this (by
adding a preprocessor definition), but the fix was broken.

This fixes the fix by:
* Adding the `conf` prefix for tool configurations (which had been
  forgotten).
* Passing `extra_b2_flags` to `boost` package to fix its build.
  * Define `BOOST_ASIO_HAS_STD_INVOKE_RESULT` in order to build boost
    1.77 with a newer compiler.

BUILD.md

@@ -258,10 +258,11 @@ part of C++20, e.g. Apple Clang 15.0,
 then you might need to add a preprocessor definition to your bulid:
 
 ```
+conan profile update 'options.boost:extra_b2_flags="define=BOOST_ASIO_HAS_STD_INVOKE_RESULT"' default
 conan profile update 'env.CFLAGS="-DBOOST_ASIO_HAS_STD_INVOKE_RESULT"' default
 conan profile update 'env.CXXFLAGS="-DBOOST_ASIO_HAS_STD_INVOKE_RESULT"' default
-conan profile update 'tools.build:cflags+=["-DBOOST_ASIO_HAS_STD_INVOKE_RESULT"]' default
-conan profile update 'tools.build:cxxflags+=["-DBOOST_ASIO_HAS_STD_INVOKE_RESULT"]' default
+conan profile update 'conf.tools.build:cflags+=["-DBOOST_ASIO_HAS_STD_INVOKE_RESULT"]' default
+conan profile update 'conf.tools.build:cxxflags+=["-DBOOST_ASIO_HAS_STD_INVOKE_RESULT"]' default
 ```