build: align minimum clang dependency with ndn-cxx

ndn-cxx requires clang 6 or later, so there's no point
supporting older versions in ndn-tools

Change-Id: I7be683abd4fce12c85187c0b5abf2ee288bbe908
diff --git a/.waf-tools/default-compiler-flags.py b/.waf-tools/default-compiler-flags.py
index 53f0f8c..7c6d282 100644
--- a/.waf-tools/default-compiler-flags.py
+++ b/.waf-tools/default-compiler-flags.py
@@ -28,9 +28,9 @@
             elif ccver < (11, 0, 0):
                 warnmsg = ('Using a version of Xcode older than 11.3 is not '
                            'officially supported and may result in build failures.')
-        elif ccver < (5, 0, 0):
+        elif ccver < (6, 0, 0):
             errmsg = ('The version of clang you are using is too old.\n'
-                      'The minimum supported clang version is 5.0.')
+                      'The minimum supported clang version is 6.0.')
         conf.flags = ClangFlags()
     else:
         warnmsg = '%s compiler is unsupported' % cxx
@@ -213,8 +213,6 @@
                               '-Wundefined-func-template',
                               '-Wno-unused-local-typedef', # Bugs #2657 and #3209
                               ]
-        if self.getCompilerVersion(conf) < (6, 0, 0):
-            flags['CXXFLAGS'] += ['-Wno-missing-braces'] # Bug #4721
         return flags
 
     def getOptimizedFlags(self, conf):
@@ -223,6 +221,4 @@
                               '-Wundefined-func-template',
                               '-Wno-unused-local-typedef', # Bugs #2657 and #3209
                               ]
-        if self.getCompilerVersion(conf) < (6, 0, 0):
-            flags['CXXFLAGS'] += ['-Wno-missing-braces'] # Bug #4721
         return flags