[GTK] Compile everything in C++11 mode

[GTK] Compile everything in C++11 mode
https://bugs.webkit.org/show_bug.cgi?id=116452

Reviewed by Anders Carlsson.

.:

* Source/autotools/SetupCompilerFlags.m4: Use the C++11 standard by default when compiling C++ source code.
Perform some minor cleanup around the comments and the order of specifying additional CXXFLAGS entries.

Source/WebKit2:

* GNUmakefile.am: Remove the -std=c++11 flags from the various CPPFLAGS lists, the mode is now specified
by default for all the C++ source code in SetupCompilerFlags.m4.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@150445 268f45cc-cd09-0410-ab3c-d52691b4dbfc