Cleanup wtf/Platform.h and config.h files

Cleanup wtf/Platform.h and config.h files
https://bugs.webkit.org/show_bug.cgi?id=83431

Reviewed by Eric Seidel.

The ENABLE() and USE() macros take care about the case when the flag
isn't defined. So there is no need to define anything with 0.

Also move duplicated code from the config.h files to Platform.h and
merge a few preprocessor commands to make the file more readable.

Source/JavaScriptCore:

* config.h:

Source/WebCore:

* config.h:

Source/WTF:

* config.h:
* wtf/Platform.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@113798 268f45cc-cd09-0410-ab3c-d52691b4dbfc