Describe the bug
vim is not _FORTIFY_SOURCE=2 clean (uses char[1] array). This causes the following problems:

• [minor] configure.ac occasionally fails to catch new way to define _FORTIFY_SOURCE=2 and users get cryptic buffer overflow crashes
• [major] gcc assumes that array are 1-byte long and can produce invalid code (similar to what glibc does on _FORTIFY_SOURCE=2 by introspecting array length)

To Reproduce
It's a downstream version of https://bugs.gentoo.org/706324 where gcc-10 was not detected and _FORTIFY_SOURCE=2 default was missed (worked around in #5580).

Expected behavior
vim should build and run on _FORTIFY_SOURCE=2 compiler.

Environment (please complete the following information):

• Vim version: 8.2.0114
• OS: Gentoo
• Terminal: alactitty-0.4.1

If vim can afford using flexible array members (https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html) it might use those:

--- a/src/structs.h
+++ b/src/structs.h
@@ -1414,7 +1414,7 @@ struct dictitem_S
 {
     typval_T	di_tv;		// type and value of the variable
     char_u	di_flags;	// flags (only used for variable)
-    char_u	di_key[1];	// key (actually longer!)
+    char_u	di_key[];	// key (actually longer!)
 };
 typedef struct dictitem_S dictitem_T;

I did not check if the rest of code does not rely on sizeof(struct dictitem_S).