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/nonVulkan.frag

@@ -2,4 +2,8 @@
 
 layout(constant_id = 17) const int arraySize = 12;            // ERROR
 layout(input_attachment_index = 1) int foo;                   // ERROR
-layout(push_constant) uniform ubn { int a; } ubi;             // ERROR
+layout(push_constant) uniform ubn { int a; } ubi;             // ERROR
+
+#ifdef VULKAN
+#error VULKAN should not be defined
+#endif

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