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/mbyte.c

@@ -1174,7 +1174,7 @@ bool utf_printable(int c)
 
 // Return true if "c" is in "table".
 static bool intable(const struct interval *table, size_t n_items, int c)
-  FUNC_ATTR_PURE
+  FUNC_ATTR_CONST
 {
   assert(n_items > 0);
   // first quick check for Latin1 etc. characters
@@ -1202,11 +1202,11 @@ static bool intable(const struct interval *table, size_t n_items, int c)
 // Return true for characters that can be displayed in a normal way.
 // Only for characters of 0x100 and above!
 bool utf_printable(int c)
-  FUNC_ATTR_PURE
+  FUNC_ATTR_CONST
 {
   // Sorted list of non-overlapping intervals.
   // 0xd800-0xdfff is reserved for UTF-16, actually illegal.
-  static struct interval nonprint[] = {
+  static const struct interval nonprint[] = {
     { 0x070f, 0x070f }, { 0x180b, 0x180e }, { 0x200b, 0x200f }, { 0x202a, 0x202e },
     { 0x2060, 0x206f }, { 0xd800, 0xdfff }, { 0xfeff, 0xfeff }, { 0xfff9, 0xfffb },
     { 0xfffe, 0xffff }