Enlarge find_other_exec's meager fgets buffer

The buffer was 100 bytes long, which is barely sufficient when the
version string gets longer (such as by configure --with-extra-version).
Set it to MAXPGPATH.

Author: Nikhil Sontakke
Discussion: https://postgr.es/m/CAMGcDxfLfpYU_Jru++L6ARPCOyxr0W+2O3Q54TDi5XdYeU36ow@mail.gmail.com

src/common/exec.c

@@ -308,7 +308,7 @@ find_other_exec(const char *argv0, const char *target,
 				const char *versionstr, char *retpath)
 {
 	char		cmd[MAXPGPATH];
-	char		line[100];
+	char		line[MAXPGPATH];
 
 	if (find_my_exec(argv0, retpath) < 0)
 		return -1;