Vulkan: Add the #define VULKAN 100 when compiled for Vulkan.

Note this requires test-based piecing together of the preamble string,
so it changed to being a std::string to make it easier to do.

This closes issue #254.

Test/vulkan.vert

@@ -37,3 +37,11 @@ void foo()
 }
 
 layout(set = 1, push_constant) uniform badpc { int a; } badpcI;  // ERROR, no descriptor set with push_constant
+
+#ifndef VULKAN
+#error VULKAN should be defined
+#endif
+
+#if VULKAN != 100
+#error VULKAN should be 100
+#endif