clang and gcc both support VLAs

Author: CamJN

src/ruby_supportlib/phusion_passenger/platform_info/compiler.rb

@@ -481,6 +481,13 @@ def self.cxx_block_support_ok?
     end
     memoize :cxx_block_support_ok?, true
 
+    def self.cxx_supports_wno_vla_cxx_extension_flag?
+      return try_compile_with_warning_flag(
+        "Checking for C++ compiler '-Wno-vla-cxx-extension' support",
+        :cxx, '', '-Wno-vla-cxx-extension')
+    end
+    memoize :cxx_supports_wno_vla_cxx_extension_flag?, true
+
     # Returns whether compiling C++ with -fvisibility=hidden might result
     # in tons of useless warnings, like this:
     # http://code.google.com/p/phusion-passenger/issues/detail?id=526

src/ruby_supportlib/phusion_passenger/platform_info/cxx_portability.rb

@@ -144,6 +144,10 @@ def self.default_extra_c_or_cxxflags(cc_or_cxx)
           # Avoids some compilaton warnings with Boost on Ubuntu 14.04.
           flags << "-Wno-unused-local-typedefs"
         end
+
+        if cxx_supports_wno_vla_cxx_extension_flag?
+          flags << "-Wno-vla-cxx-extension"
+        end
       else
         flags << debugging_cflags
       end