From upstream: https://chiselapp.com/user/moinejf/repository/abcm2ps/info/61ba7bd35d5ad451

Index: format.c
==================================================================
--- a/format.c
+++ b/format.c
@@ -11,10 +11,11 @@
  * by the Free Software Foundation; either version 3 of the License, or
  * (at your option) any later version.
  */
 
 #include <stdlib.h>
+#include <stdbool.h>
 #include <string.h>
 #include <ctype.h>
 
 #include "abcm2ps.h"
 
@@ -1158,17 +1159,17 @@
 				cfmt.fields[0] &= ~(1 << ('X' - 'A'));
 			return;
 		}
 		if (strcmp(w, "writehistory") == 0) {	/* compatibility */
 			struct SYMBOL *s;
-			int bool;
+			bool boolv;
 			unsigned u;
 
-			bool = get_bool(p);
+			boolv = get_bool(p);
 			for (s = info['I' - 'A']; s != 0; s = s->next) {
 				u = s->text[0] - 'A';
-				if (bool)
+				if (boolv)
 					cfmt.fields[0] |= (1 << u);
 				else
 					cfmt.fields[0] &= ~(1 << u);
 			}
 			return;
@@ -1215,19 +1216,19 @@
 				else
 					deco['M'] = "lowermordent";
 			}
 			break;
 		case 1:	{			/* %%writefields */
-			int bool;
+			bool boolv;
 			unsigned u;
 
 			q = p;
 			while (*p != '\0' && !isspace((unsigned char) *p))
 				p++;
 			while (isspace((unsigned char) *p))
 				p++;
-			bool = get_bool(p);
+			boolv = get_bool(p);
 			while (*q != '\0' && !isspace((unsigned char) *q)) {
 				u = *q - 'A';
 				if (u < 26) {
 					i = 0;
 				} else {
@@ -1235,11 +1236,11 @@
 					if (u < 26)
 						i = 1;
 					else
 						break;	/*fixme: error */
 				}
-				if (bool)
+				if (boolv)
 					cfmt.fields[i] |= (1 << u);
 				else
 					cfmt.fields[i] &= ~(1 << u);
 				q++;
 			}