doc: update configure to require g++ 4.9.4

PR-URL: #14204
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

configure

@@ -651,8 +651,8 @@ def check_compiler(o):
   ok, is_clang, clang_version, gcc_version = try_check_compiler(CXX, 'c++')
   if not ok:
     warn('failed to autodetect C++ compiler version (CXX=%s)' % CXX)
-  elif clang_version < '3.4.2' if is_clang else gcc_version < '4.8.0':
-    warn('C++ compiler too old, need g++ 4.8 or clang++ 3.4.2 (CXX=%s)' % CXX)
+  elif clang_version < '3.4.2' if is_clang else gcc_version < '4.9.4':
+    warn('C++ compiler too old, need g++ 4.9.4 or clang++ 3.4.2 (CXX=%s)' % CXX)
 
   ok, is_clang, clang_version, gcc_version = try_check_compiler(CC, 'c')
   if not ok: