uuid no longer packable with gcc

[wkelton]

There appears to be a breaking change for uuid in version 1.86 where it cannot be packed in a struct using gcc.

I set up an example showing a 1.85 uuid being packed in a struct, but 1.86 uuid causes a warning from gcc: Compile Explorer

gcc warns:

warning: ignoring packed attribute because of unpacked non-POD field

[pdimov]

I wasn't able to make __attribute__((packed)) work reliably after the changes; you'll have to define BOOST_UUID_DISABLE_ALIGNMENT if you need this functionality. (#pragma pack(1) works, though.)

The corresponding tests are

https://github.com/boostorg/uuid/blob/02c82cea9c82e4fdbc32b58e5a2e9f97dfc50755/test/test_attribute_packed.cpp
(which requires BOOST_UUID_DISABLE_ALIGNMENT to pass) and

https://github.com/boostorg/uuid/blob/02c82cea9c82e4fdbc32b58e5a2e9f97dfc50755/test/test_pragma_pack.cpp

which does not.