[boost] [Config] proposal of new macro, BOOST_DECL

In order to export class from dynamic library one needs to add something as: __declspec(dllexport) wrapped in a macro. (Win32 example.) Right now libraries define their own macros, e.g Iostreams has BOOST_IO_DECL and Regex uses BOOST_REGEX_DECL. There could be more. I suggest common macro, BOOST_DECL to be used Boost-wide. Its value may be by default empty or deduced from BOOST_ALL_DYN_LINK as it is done in Regex. /Pavel