Fix a few build issues seen by various compilers

that I have on my system but haven't used for a while
(a couple of warnings and a couple of errors)

src/files.c

@@ -45,7 +45,7 @@ extern int errno;
 #include <stdlib.h>
 #endif
 
-#if defined(UNIX) || defined(VMS)
+#if defined(UNIX) || defined(VMS) || defined(_MSC_VER)
 #include <signal.h>
 #endif
 

src/mail.c

@@ -440,7 +440,7 @@ struct obj *otmp;
 
 	/* XXX replace with more general substitution code and add local
 	 * contact message.  Also use DEVTEAM_URL */
-    if(junk[0]) == NULL){
+    if(junk[0] == NULL){
 #define BUGS_FORMAT "Report bugs to %s."
       junk[0] = (char *)alloc(strlen(BUGS_FORMAT) + strlen(DEVTEAM_EMAIL));
       sprintf(junk[0], DEVTEAM_EMAIL);

src/trap.c

@@ -2399,7 +2399,7 @@ glovecheck:		    target = which_armor(mtmp, W_ARMG);
 			    if (in_sight) seetrap(trap);
 			    if ((mtmp->mhp -= dmgval2) <= 0)
 				monkilled(mtmp, in_sight ?
-				    "compression from an anti-magic field" : 0,
+				    "compression from an anti-magic field" : (const char *)0,
 					  -AD_MAGM);
 			    if (mtmp->mhp <= 0) trapkilled = TRUE;
 			    if (see_it) newsym(trap->tx, trap->ty);

win/share/tile.h

@@ -1,6 +1,6 @@
 typedef unsigned char pixval;
 
-typedef struct {
+typedef struct pixel_t {
     pixval r, g, b;
 } pixel;