Add cvars.h header

With macros for declaring cvar_t ptrs in source and header, registering
new cvars and checking if the value is positive.

Also cvars_init function and cv_bhop declaration

src/include/cvars.h

@@ -0,0 +1,38 @@
+
+#ifndef _CVARS_H
+#define _CVARS_H
+
+#include "sdk.h"
+#include "globals.h"
+
+#define CVAR_PREFIX  "cv_"
+#define CVAR_HACK_ID 0x2000 /* (1<<13) */
+
+/*
+ *  DECL_CVAR: Declares cvar variable in source file.
+ *  DECL_CVAR_EXTERN: Same but for headers.
+ *  REGISTER_CVAR: Create the cvar, return cvar_t*
+ *  CVAR_ON: Returns true if the cvar is non-zero
+ *
+ * prefix | meaning
+ * -------+-------------------------------
+ * cv_*   | cvar variable
+ */
+#define DECL_CVAR(name) cvar_t* cv_##name = NULL;
+
+#define DECL_CVAR_EXTERN(name) extern cvar_t* cv_##name;
+
+#define REGISTER_CVAR(name, value) \
+    i_engine->pfnRegisterVariable(CVAR_PREFIX name, value, CVAR_HACK_ID);
+
+#define CVAR_ON(name) (cv_##name->value != 0)
+
+/*----------------------------------------------------------------------------*/
+
+DECL_CVAR_EXTERN(bhop);
+
+/*----------------------------------------------------------------------------*/
+
+bool cvars_init(void);
+
+#endif /* _CVARS_H */