[GTK] Limit the supported compilers to GCC >= 4.7 and Clang >= 3.0
https://bugs.webkit.org/show_bug.cgi?id=109932

Patch by Zan Dobersek <zdobersek@igalia.com> on 2013-03-07
Reviewed by Martin Robinson.

.: 

* Source/autotools/CheckSystemAndBasicDependencies.m4: Strictly check for one of the two compilers.
* Source/autotools/SetupCompilerFlags.m4: Set up C++11-specific compiler flags. Force the C99 standard for the C code.
Use the -Qunused-arguments flag when using Clang, it reduces irrelevant errors when using ccache.

Source/WebCore: 

* GNUmakefile.am: Replace references to SYMBOL_VISIBILITY_INLINES and SYMBOL_VISIBILITY variables with the actual flags.
They are now available by default due to the limited set of supported compilers.
* bindings/gobject/GNUmakefile.am: Ditto.

Source/WebKit/gtk: 

* GNUmakefile.am: Replace references to SYMBOL_VISIBILITY_INLINES and SYMBOL_VISIBILITY variables with the actual flags.
They are now available by default due to the limited set of supported compilers.

Source/WebKit2: 

* GNUmakefile.am: Replace references to SYMBOL_VISIBILITY_INLINES and SYMBOL_VISIBILITY variables with the actual flags.
They are now available by default due to the limited set of supported compilers.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@145084 268f45cc-cd09-0410-ab3c-d52691b4dbfc