refactor: use flexible arrays instead of the length-of-one trick (#22072)

The "length-of-one" trick, where the last element of a struct is an
array of size 1, but extra size is allocated when calling malloc where
it uses more than 1 element in the array, cause problems with some
compilers. Some compilers set _FORTIFY_SOURCE=2 by default which
incorrectly considers it as an overflow. More information:
https://github.com/neovim/neovim/issues/223#issuecomment-1413828554

Using flexible array members allows us to to properly convey to the
compiler that its size may be larger than 1. This also enables us to
remove lengthy workarounds that are unreliable, as they depend on
CMAKE_BUILD_TYPE which isn't defined for multi-config generators.

Closes: https://github.com/neovim/neovim/issues/223

src/nvim/sign_defs.h

@@ -12,7 +12,7 @@
 typedef struct signgroup_S {
   int sg_next_sign_id;   ///< next sign id for this group
   uint16_t sg_refcount;  ///< number of signs in this group
-  char sg_name[1];       ///< sign group name, actually longer
+  char sg_name[];        ///< sign group name
 } signgroup_T;
 
 // Macros to get the sign group structure from the group name