fmemopen.c: portability fix around funopen()

As noted in main.c, funopen() is used when fmemopen() is not available.
Unfortunately, funopen() is not portable and its definition is not
stable across Operating Systems (Darwin/macOS uses fpos_t while NetBSD
uses off_t).

This change fixes the build on NetBSD, where fpos_t is an opaque type
and does not support arithmetic. To be fair, even macOS' manual page for
funopen() mentions it erroneously assumes that fpos_t is an integral
type, and invites to read fseek(3) for a discussion of this issue.
Therefore, arguably the default should be off_t instead, but I did not
want to possibly break functionality anywhere else.

Author: khorben

fmemopen.c

@@ -20,6 +20,12 @@
 #include <string.h>
 #include <sys/mman.h>
 
+#if defined(__NetBSD__)
+# define FPOS_T off_t
+#else
+# define FPOS_T fpos_t
+#endif
+
 struct fmem {
   size_t pos;
   size_t size;
@@ -53,7 +59,7 @@ static int writefn(void *handler, const char *buf, int size) {
   return size;
 }
 
-static fpos_t seekfn(void *handler, fpos_t offset, int whence) {
+static FPOS_T seekfn(void *handler, FPOS_T offset, int whence) {
   size_t pos;
   fmem_t *mem = handler;
 
@@ -83,7 +89,7 @@ static fpos_t seekfn(void *handler, fpos_t offset, int whence) {
   }
 
   mem->pos = pos;
-  return (fpos_t)pos;
+  return (FPOS_T)pos;
 }
 
 static int closefn(void *handler) {