eval: add fname_case to feature list

runtime/doc/eval.txt

@@ -5125,6 +5125,8 @@ has({feature})	Returns 1 if {feature} is supported, 0 otherwise.  The
 		        iconv		Can use |iconv()| for conversion.
 		        +shellslash	Can use backslashes in filenames (Windows)
 			clipboard	|clipboard| provider is available.
+			fname_case	Case in file names matters (for Darwin and MS-Windows
+					this is not present).
 			mac		MacOS system.
 			nvim		This is Nvim.
 			python2		Legacy Vim |python2| interface. |has-python|

src/nvim/eval/funcs.c

@@ -4131,7 +4131,9 @@ static void f_has(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 #ifdef _WIN64
     "win64",
 #endif
+#ifndef CASE_INSENSITIVE_FILENAME
     "fname_case",
+#endif
 #ifdef HAVE_ACL
     "acl",
 #endif