Work around structure size / packing issue on 64 bit Intel Mac builds. Long

comment explaining the patch in the patch.

Refs trac:574

This commit was SVN r13276.

The following Trac tickets were found above:
  Ticket 574 --> https://svn.open-mpi.org/trac/ompi/ticket/574

opal/util/if.c

@@ -41,15 +41,22 @@
 #include <arpa/inet.h>
 #endif
 #ifdef HAVE_NET_IF_H
-#if defined(__APPLE__) && defined(__POWERPC__) && defined(_LP64)
-/* Apple engineering suggested this as a workaround for a bug in OS X
- * 10.4 (Tiger) that prevented ioctl(..., SIOCGIFCONF, ...) from
- * working properly in 64 bit mode */
-#pragma options align=power
+#if defined(__APPLE__) && defined(_LP64)
+/* Apple engineering suggested using options align=power as a
+   workaround for a bug in OS X 10.4 (Tiger) that prevented ioctl(...,
+   SIOCGIFCONF, ...) from working properly in 64 bit mode on Power PC.
+   It turns out that the underlying issue is the size of struct
+   ifconf, which the kernel expects to be 12 and natural 64 bit
+   alignment would make 16.  The same bug appears in 64 bit mode on
+   Intel macs, but align=power is a no-op there, so instead, use the
+   pack pragma to instruct the compiler to pack on 4 byte words, which
+   has the same effect as align=power for our needs and works on both
+   Intel and Power PC Macs. */
+#pragma pack(push,4)
 #endif
 #include <net/if.h>
-#if defined(__APPLE__) && defined(__POWERPC__) && defined(_LP64)
-#pragma options align=reset
+#if defined(__APPLE__) && defined(_LP64)
+#pragma pack(pop)
 #endif
 #endif
 #ifdef HAVE_NETDB_H