refactor(build): don't use the preprocessor when generating headers

Using the preprocessor before generating prototypes provides some
"niceties" but the places that rely on these are pretty few.
Vastly simplifying the BUILD SYSTEM is a better trade-off.

Unbalancing { } blocks due to the preprocessor is cringe anyway (think
of the tree-sitter trees!), write these in a different way.

Add some workarounds for plattform specific features.

INCLUDE_GENERATED_DECLARATIONS flag is now technically redundant,
but will be cleaned up in a follow-up PR as it is a big mess.

src/nvim/main.h

@@ -47,6 +47,11 @@ typedef struct {
   bool had_stdin_file;                  // explicit - as a file to edit
 } mparm_T;
 
+#if defined(MSWIN) && !defined(ENABLE_ASAN_UBSAN)
+# define __asan_default_options vim__asan_default_options
+# define __ubsan_default_options vim__ubsan_default_options
+#endif
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "main.h.generated.h"
 #endif