objc tests: Disable nsstring test on Clang + Linux

The GNUstep runtime shipped in Linux distros is GCC-specific and won't
work with clang. You have to build it separately and set the paths
yourself.

In file included from /root/b 05ywf6dg/stringprog@exe/stringprog-unity.m:1:
In file included from /root/b 05ywf6dg/../test cases/objc/2 nsstring/stringprog.m:1:
In file included from /usr/include/GNUstep/Foundation/NSString.h:89:
In file included from /usr/include/GNUstep/Foundation/NSObject.h:30:
In file included from /usr/include/GNUstep/Foundation/NSObjCRuntime.h:213:
/usr/include/GNUstep/GNUstepBase/GSObjCRuntime.h:58:11: fatal error: 'objc/objc.h' file not found
 #include <objc/objc.h>
          ^

On my system, I get the same error and the objc.h path is:

/usr/lib/gcc/x86_64-redhat-linux/6.3.1/include/objc/objc.h

test cases/objc/2 nsstring/meson.build

@@ -4,6 +4,9 @@ if host_machine.system() == 'darwin'
   dep = dependency('appleframeworks', modules : 'foundation')
 else
   dep = dependency('gnustep')
+  if host_machine.system() == 'linux' and meson.get_compiler('objc').get_id() == 'clang'
+    error('MESON_SKIP_TEST: GNUstep is broken on Linux with Clang')
+  endif
 endif
 exe = executable('stringprog', 'stringprog.m', dependencies : dep)
 test('stringtest', exe)