Make SHM_ALL to a variable instead of a compound literal #define

gcc-9 has [improved compliance] with the C spec for lifetime of compound
literals, tying their lifetime to block scope instead of function scope.
This makes the behavior comparable to all other automatic variables.

Using the SHM_ALL #define instantiated a compound literal local to an if
clause and assigned the address to a "char_u *".  Since the pointer was
then being used outside of the if clause, it was using an invalid
address.

[improved compliance]: https://gcc.gnu.org/gcc-9/porting_to.html#complit

Closes #9855

src/nvim/option.c

@@ -306,6 +306,15 @@ static char *(p_cot_values[]) =       { "menu", "menuone", "longest", "preview",
 static char *(p_icm_values[]) =       { "nosplit", "split", NULL };
 static char *(p_scl_values[]) =       { "yes", "no", "auto", NULL };
 
+/// All possible flags for 'shm'.
+static char_u SHM_ALL[] = {
+  SHM_RO, SHM_MOD, SHM_FILE, SHM_LAST, SHM_TEXT, SHM_LINES, SHM_NEW, SHM_WRI,
+  SHM_ABBREVIATIONS, SHM_WRITE, SHM_TRUNC, SHM_TRUNCALL, SHM_OVER,
+  SHM_OVERALL, SHM_SEARCH, SHM_ATTENTION, SHM_INTRO, SHM_COMPLETIONMENU,
+  SHM_RECORDING, SHM_FILEINFO,
+  0,
+};
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "option.c.generated.h"
 #endif

src/nvim/option_defs.h

@@ -178,14 +178,6 @@ enum {
   SHM_RO, SHM_MOD, SHM_FILE, SHM_LAST, SHM_TEXT, SHM_LINES, SHM_NEW, SHM_WRI, \
   0, \
 })
-/// All possible flags for 'shm'.
-#define SHM_ALL ((char_u[]) { \
-  SHM_RO, SHM_MOD, SHM_FILE, SHM_LAST, SHM_TEXT, SHM_LINES, SHM_NEW, SHM_WRI, \
-  SHM_ABBREVIATIONS, SHM_WRITE, SHM_TRUNC, SHM_TRUNCALL, SHM_OVER, \
-  SHM_OVERALL, SHM_SEARCH, SHM_ATTENTION, SHM_INTRO, SHM_COMPLETIONMENU, \
-  SHM_RECORDING, SHM_FILEINFO, \
-  0, \
-})
 
 /* characters for p_go: */
 #define GO_ASEL         'a'             /* autoselect */