As for libc's `system', change Awk to look for `sh' in $PATH.  This patch is
only meant to be used during bootstrapping, where we don't know in advance the
absolute file name of `sh'.

--- gawk-4.1.3/io.c	2015-05-19 15:37:20.000000000 +0200
+++ gawk-4.1.3/io.c	2015-06-09 18:39:36.918414881 +0200
@@ -2025,7 +2025,7 @@
 
 		set_sigpipe_to_default();
 
-		execl("/bin/sh", "sh", "-c", command, NULL);
+		execlp("sh", "sh", "-c", command, NULL);
 		_exit(errno == ENOENT ? 127 : 126);
 
 	case -1:
@@ -2077,7 +2077,7 @@
 
 		signal(SIGPIPE, SIG_DFL);
 
-		execl("/bin/sh", "sh", "-c", command, NULL);
+		execlp("sh", "sh", "-c", command, NULL);
 		_exit(errno == ENOENT ? 127 : 126);
 
 	case -1:
@@ -2422,7 +2422,7 @@
 			fatal(_("close of pipe failed (%s)"), strerror(errno));
 		/* stderr does NOT get dup'ed onto child's stdout */
 		set_sigpipe_to_default();
-		execl("/bin/sh", "sh", "-c", str, NULL);
+		execlp("sh", "sh", "-c", str, NULL);
 		_exit(errno == ENOENT ? 127 : 126);
 	}
 #endif /* NOT __EMX__, NOT __MINGW32__ */
@@ -2659,7 +2659,7 @@
 		if (close(p[0]) == -1 || close(p[1]) == -1)
 			fatal(_("close of pipe failed (%s)"), strerror(errno));
 		set_sigpipe_to_default();
-		execl("/bin/sh", "sh", "-c", cmd, NULL);
+		execlp("sh", "sh", "-c", cmd, NULL);
 		_exit(errno == ENOENT ? 127 : 126);
 	}
 #endif /* NOT __EMX__, NOT __MINGW32__ */