Implement getpagesize() for djgpp

This is the "correct" way to find the page size on DPMI.  In practice
though, this could be hardcoded to return 4096 and no one would ever
notice the difference.

src/os.cc

@@ -51,6 +51,10 @@
 #  include <windows.h>
 #endif
 
+#ifdef DJGPP
+#  include <dpmi.h>
+#endif
+
 #ifdef fileno
 #  undef fileno
 #endif
@@ -354,6 +358,11 @@ long getpagesize() {
   SYSTEM_INFO si;
   GetSystemInfo(&si);
   return si.dwPageSize;
+#  elif defined(DJGPP)
+  unsigned long size;
+  if (__dpmi_get_page_size(&size) != 0)
+    FMT_THROW(system_error(ENOSYS, "cannot get memory page size"));
+  return size;
 #  else
   long size = FMT_POSIX_CALL(sysconf(_SC_PAGESIZE));
   if (size < 0) FMT_THROW(system_error(errno, "cannot get memory page size"));