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/.waf-tools/default-compiler-flags.py b/.waf-tools/default-compiler-flags.py
index 0bf48cc..53f0f8c 100644
--- a/.waf-tools/default-compiler-flags.py
+++ b/.waf-tools/default-compiler-flags.py
@@ -16,20 +16,21 @@
     errmsg = ''
     warnmsg = ''
     if cxx == 'gcc':
-        if ccver < (5, 3, 0):
+        if ccver < (7, 4, 0):
             errmsg = ('The version of gcc you are using is too old.\n'
-                      'The minimum supported gcc version is 7.4.0.')
-        elif ccver < (7, 4, 0):
-            warnmsg = ('Using a version of gcc older than 7.4.0 is not '
-                       'officially supported and may result in build failures.')
+                      'The minimum supported gcc version is 7.4.')
         conf.flags = GccFlags()
     elif cxx == 'clang':
-        if Utils.unversioned_sys_platform() == 'darwin' and ccver < (9, 0, 0):
-            errmsg = ('The version of Xcode you are using is too old.\n'
-                      'The minimum supported Xcode version is 9.0.')
-        elif ccver < (4, 0, 0):
+        if Utils.unversioned_sys_platform() == 'darwin':
+            if ccver < (10, 0, 0):
+                errmsg = ('The version of Xcode you are using is too old.\n'
+                          'The minimum supported Xcode version is 11.3.')
+            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):
             errmsg = ('The version of clang you are using is too old.\n'
-                      'The minimum supported clang version is 4.0.')
+                      'The minimum supported clang version is 5.0.')
         conf.flags = ClangFlags()
     else:
         warnmsg = '%s compiler is unsupported' % cxx
@@ -181,7 +182,7 @@
         flags['CXXFLAGS'] += ['-fdiagnostics-color',
                               '-Wredundant-tags',
                               ]
-        if platform.machine() == 'armv7l' and self.getCompilerVersion(conf) >= (7, 1, 0):
+        if platform.machine() == 'armv7l':
             flags['CXXFLAGS'] += ['-Wno-psabi'] # Bug #5106
         return flags
 
@@ -190,7 +191,7 @@
         flags['CXXFLAGS'] += ['-fdiagnostics-color',
                               '-Wredundant-tags',
                               ]
-        if platform.machine() == 'armv7l' and self.getCompilerVersion(conf) >= (7, 1, 0):
+        if platform.machine() == 'armv7l':
             flags['CXXFLAGS'] += ['-Wno-psabi'] # Bug #5106
         return flags