Improve platform detection and reduce includes:

The existing platform detection code was derived from the old Beast
library, which was, itself, derived from JUCE.

This commit removes that code and replaces it with the Boost.Predef
library which defines a consistent set of compiler, architecture,
operating system, library, and other version numbers.

For more on Boost.Predef, please see the Boost documentation. The
documentation for the current version as of this writing is at:
https://www.boost.org/doc/libs/1_71_0/doc/html/predef.html

src/ripple/app/misc/NetworkOPs.cpp

@@ -1084,7 +1084,7 @@ void NetworkOPsImp::apply (std::unique_lock<std::mutex>& batchLock)
             if (isTemMalformed (e.result))
                 app_.getHashRouter().setFlags (e.transaction->getID(), SF_BAD);
 
-    #ifdef BEAST_DEBUG
+    #ifdef DEBUG
             if (e.result != tesSUCCESS)
             {
                 std::string token, human;