[ITK] clang throws a lot of -Wunknown-attributes warning.

[Poughon Victor]

Hi Matt,

Here is a dashboard build with the warnings we are seeing with Rashad:
https://open.cdash.org/viewBuildError.php?type=1&buildid=4123272

Compiler:
clang version 3.5.0 (tags/RELEASE_350/final)
Target: x86_64-redhat-linux-gnu
Thread model: posix

CXX_FLAGS:
-Wall

OS:
Fedora release 22
Linux pc-christophe.cst.cnes.fr 4.2.3-200.fc22.x86_64 #1 SMP Thu Oct 8 03:23:55 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux

Apparently there could be a "!defined(__clang__)" missing in itkStaticAssert.h when testing for gcc.

Let me know if you need more info,

Victor Poughon

> Hi Rashad,
> 
> Thanks for discussing issue.
> 
> I have been using that compiler and have not come across those warnings.
> Are there any non-default flags or CMake options being used?
> 
> Could you please submit an Experimental build to the dashboard? ('ctest -D
> Experimental' from a clean build tree)
> 
> Thanks,
> Matt