Add an option to undefine __IMAGE_SUPPORT__ (#353)

Clang now defines it by default (was picked up in the update)
and the conformance tests check that this is defined if and only
if the runtime advertises support.

Signed-off-by: Kévin Petit <kpet@free.fr>

diff --git a/lib/Option.cpp b/lib/Option.cpp
index a0d929b..ec04e7e 100644
--- a/lib/Option.cpp
+++ b/lib/Option.cpp
@@ -153,6 +153,9 @@
 static llvm::cl::opt<bool>
     cplusplus("c++", llvm::cl::init(false),
               llvm::cl::desc("Enable experimental C++ support"));
+
+static llvm::cl::opt<bool> images("images", llvm::cl::init(true),
+                                  llvm::cl::desc("Enable support for images"));
 } // namespace
 
 namespace clspv {
@@ -185,6 +188,7 @@
 bool KeepUnusedArguments() { return keep_unused_arguments; }
 bool Int8Support() { return int8_support; }
 bool CPlusPlus() { return cplusplus; }
+bool ImageSupport() { return images; }
 
 } // namespace Option
 } // namespace clspv