Old compiler can't use "string"[0]

nanovna.h

@@ -668,6 +668,8 @@ extern const uint8_t numfont16x22[];
 #define S_INFINITY "\031"  // 0x19
 #define S_LARROW   "\032"  // 0x1A
 #define S_RARROW   "\033"  // 0x1B
+#define C_LARROW   0x1A  // 0x1A
+#define C_RARROW   0x1B  // 0x1B
 #define S_PI       "\034"  // 0x1C
 #define S_MICRO    "\035"  // 0x1D
 #define S_OHM      "\036"  // 0x1E

ui.c

@@ -1213,7 +1213,7 @@ static const keypads_t keypads_text[] = {
   {0x00, '1'}, {0x10, '2'}, {0x20, '3'}, {0x30, '4'}, {0x40, '5'}, {0x50, '6'}, {0x60, '7'}, {0x70, '8'}, {0x80, '9'}, {0x90, '0'},
   {0x01, 'Q'}, {0x11, 'W'}, {0x21, 'E'}, {0x31, 'R'}, {0x41, 'T'}, {0x51, 'Y'}, {0x61, 'U'}, {0x71, 'I'}, {0x81, 'O'}, {0x91, 'P'},
   {0x02, 'A'}, {0x12, 'S'}, {0x22, 'D'}, {0x32, 'F'}, {0x42, 'G'}, {0x52, 'H'}, {0x62, 'J'}, {0x72, 'K'}, {0x82, 'L'}, {0x92, '_'},
-  {0x03, '-'}, {0x13, 'Z'}, {0x23, 'X'}, {0x33, 'C'}, {0x43, 'V'}, {0x53, 'B'}, {0x63, 'N'}, {0x73, 'M'}, {0x83, S_LARROW[0]}, {0x93, S_SARROW[0]},
+  {0x03, '-'}, {0x13, 'Z'}, {0x23, 'X'}, {0x33, 'C'}, {0x43, 'V'}, {0x53, 'B'}, {0x63, 'N'}, {0x73, 'M'}, {0x83, C_LARROW}, {0x93, C_RARROW},
 };
 
 enum {