Fixes these warnings:

endfile.c:30:27: warning: format specifies type 'long' but the argument has type 'ftnint' (aka 'int') [-Wformat]
   30 |                 sprintf(nbuf,"fort.%ld",a->aunit);
      |                                    ~~~  ^~~~~~~~
      |                                    %d

open.c:285:32: warning: format specifies type 'long' but the argument has type 'ftnint' (aka 'int') [-Wformat]
  285 |       sprintf(buf, "fort.%ld", a->ounit);
      |                          ~~~   ^~~~~~~~
      |                          %d

open.c:434:35: warning: format specifies type 'long' but the argument has type 'ftnint' (aka 'int') [-Wformat]
  434 |    (void) sprintf(nbuf,"fort.%ld",n);
      |                              ~~~  ^
      |                              %d

signal_.c:14:9: warning: cast to smaller integer type 'ftnint' (aka 'int') from 'void (*)(int)' [-Wpointer-to-int-cast]
   14 |         return (ftnint)signal(sig, proc);
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~

diff -ur src/cspice/endfile.c src/cspice/endfile.c
--- src/cspice/endfile.c
+++ src/cspice/endfile.c
@@ -26,8 +26,8 @@
 	if(a->aunit>=MXUNIT || a->aunit<0) err(a->aerr,101,"endfile");
 	b = &f__units[a->aunit];
 	if(b->ufd==NULL) {
-		char nbuf[10];
-		sprintf(nbuf,"fort.%ld",a->aunit);
+		char nbuf[32];
+		sprintf(nbuf,"fort.%ld",(long)a->aunit);
 		if (tf = fopen(nbuf, f__w_mode[0]))
 			fclose(tf);
 		return(0);
diff -ur src/cspice/open.c src/cspice/open.c
--- src/cspice/open.c
+++ src/cspice/open.c
@@ -282,7 +282,7 @@
          opnerr(a->oerr,107,"open")
       }
    else
-      sprintf(buf, "fort.%ld", a->ounit);
+      sprintf(buf, "fort.%ld", (long)a->ounit);
    b->uscrtch = 0;
    b->uend=0;
    b->uwrt = 0;
@@ -429,9 +429,9 @@
 
 #endif
    {
-   char nbuf[10];
+   char nbuf[32];
    olist a;
-   (void) sprintf(nbuf,"fort.%ld",n);
+   (void) sprintf(nbuf,"fort.%ld",(long)n);
    a.oerr=1;
    a.ounit=n;
    a.ofnm=nbuf;
diff -ur src/cspice/signal_.c src/cspice/signal_.c
--- src/cspice/signal_.c
+++ src/cspice/signal_.c
@@ -1,7 +1,7 @@
 #include "f2c.h"
 #include "signal1.h"
 
- ftnint
+ sig_pf
 #ifdef KR_headers
 signal_(sigp, proc) integer *sigp; sig_pf proc;
 #else
@@ -11,5 +11,5 @@
 	int sig;
 	sig = (int)*sigp;
 
-	return (ftnint)signal(sig, proc);
+	return signal(sig, proc);
 	}