build: raise minimum build requirements

 * Make gcc 7.4 and boost 1.65.1 hard dependencies, both are
   available out-of-the-box on Ubuntu 18.04
 * Require Xcode 10 on macOS but recommend 11.3 or later
 * If using clang on non-Apple platforms, the minimum supported
   version is now 6.0

Change-Id: I79c84a266741c9a66655da4fd467fe0d8218e8f0
diff --git a/wscript b/wscript
index f9213f2..e7146d8 100644
--- a/wscript
+++ b/wscript
@@ -126,14 +126,10 @@
         boost_libs.append('unit_test_framework')
 
     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')
 
     # Workaround for bug 4860
     if conf.env.BOOST_VERSION_NUMBER < 106900 and conf.env.CXX_NAME == 'clang':