Utility: blind attempt to fix compilation on Apple's Clang "4.2".

It's Clang 3.2-svn and the libc++ (some prerelease, because it was first
released for 3.3) doesn't have constexpr operator bool for
std::integral_constant, making debug operator for iterable types
noncompilable.
Utility: another blind attempt to fix compilation with Apple Clang "4.2".

Forgot that Utility::IsIterable is also std::integral_constant.
Doc++
Include configure.h from Corrade.h.

So the user really gets all configured preprocessor variables when
including <Corrade/Corrade.h>, as stated in many places in the docs.
Merge branch 'master' into compatibility

Conflicts:
	doc/building-corrade.dox
	src/Corrade/Corrade.h
	src/Corrade/Utility/Debug.h
doc: crosslink preprocessor variables with CMake documentation.
Merge branch 'master' into compatibility
doc: crosslink also compatibility preprocessor vars with CMake docs.