build: raise minimum build requirements

 * Hard-depend on gcc >= 7.4 and clang >= 5.0, which have
   almost complete C++17 support
 * Require Xcode 10 on macOS but recommend 11.3 or later
 * Boost >= 1.65.1 is now a hard requirement

Change-Id: I76eb915b226e8f5dd5c5fdbde9b23befc2a37c90
diff --git a/wscript b/wscript
index f0c4d6d..92b710c 100644
--- a/wscript
+++ b/wscript
@@ -36,14 +36,10 @@
         conf.define('WITH_TESTS', 1)
 
     conf.check_boost(lib=boost_libs, mt=True)
-    if conf.env.BOOST_VERSION_NUMBER < 105800:
+    if conf.env.BOOST_VERSION_NUMBER < 106501:
         conf.fatal('The minimum supported version of Boost is 1.65.1.\n'
                    'Please upgrade your distribution or manually install a newer version of Boost.\n'
                    'For more information, see https://redmine.named-data.net/projects/nfd/wiki/Boost')
-    elif conf.env.BOOST_VERSION_NUMBER < 106501:
-        Logs.warn('WARNING: Using a version of Boost older than 1.65.1 is not officially supported and may not work.\n'
-                  'If you encounter any problems, please upgrade your distribution or manually install a newer version of Boost.\n'
-                  'For more information, see https://redmine.named-data.net/projects/nfd/wiki/Boost')
 
     conf.recurse('tools')