build: warn for gcc versions earlier than 8.3.0

Update the warning threshold to match the minimum supported version
of gcc. Builds can still proceed, but are not guaranteed to work.

configure.py

@@ -960,8 +960,8 @@ def check_compiler(o):
                 ('clang ' if is_clang else '', CXX, version_str))
   if not ok:
     warn('failed to autodetect C++ compiler version (CXX=%s)' % CXX)
-  elif clang_version < (8, 0, 0) if is_clang else gcc_version < (6, 3, 0):
-    warn('C++ compiler (CXX=%s, %s) too old, need g++ 6.3.0 or clang++ 8.0.0' %
+  elif clang_version < (8, 0, 0) if is_clang else gcc_version < (8, 3, 0):
+    warn('C++ compiler (CXX=%s, %s) too old, need g++ 8.3.0 or clang++ 8.0.0' %
          (CXX, version_str))
 
   ok, is_clang, clang_version, gcc_version = try_check_compiler(CC, 'c')