Set an upper range of compiler versions for -Werror

This allows us to go back to older versions of Qt with newer compilers,
that didn't exist when those versions were released. It also allows
someone upgrading their compiler and not being faced with having to fix
all warnings before Qt compiles.

This commit whitelists the following compilers:
* Apple Clang versions 4.0, 4.1 and 4.2 (OS X only)
* Intel Compiler versions 13.0, 13.1 and 14.0 (Linux only)
* GCC versions 4.6, 4.7 and 4.8 (all OS)

Notably, Clang on other other OS besides OS X and MSVC are missing.

Change-Id: I665160d40a59336da1904f2a6c1eda543e592b48
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>