preproc: error out if an include file exists but can't be opened

If an include file exists, but cannot be opened, that is still a
critical error.

However, downgrade this from a fatal to a nonfatal error. There really
isn't any reason to stop cold here.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>

asm/preproc.c

@@ -2241,12 +2241,15 @@ static FILE *inc_fopen(const char *file,
             strlist_add(dhead, file);
     }
 
-    if (!path) {
-        if (omode == INC_NEEDED)
-            nasm_fatal("unable to open include file `%s'", file);
-    } else {
-        if (!fp && omode != INC_PROBE)
+    if (path && !fp && omode != INC_PROBE)
         fp = nasm_open_read(path, fmode);
+
+    if (omode == INC_NEEDED && !fp) {
+        if (!path)
+            errno = ENOENT;
+
+        nasm_nonfatal("unable to open include file `%s': %s",
+                      file, strerror(errno));
     }
 
     if (found_path)

include/compiler.h

@@ -88,6 +88,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <limits.h>
+#include <errno.h>
 
 #ifdef HAVE_STRINGS_H
 # include <strings.h>

test/include-self.asm

@@ -0,0 +1,2 @@
+; Test an infinite %include loop
+%include "include-self.asm"