Warning whe compiling with -Wundef on clang

[viboes]

When compiling with -Wundef on clang we get the following warning

/xxx/modular-boost3/boost/type_traits/is_nothrow_move_constructible.hpp:30:79: warning: 'BOOST_GCC_VERSION_WORKAROUND_GUARD' is not defined, evaluates to 0 [-Werror,-Wundef]
#elif !defined(BOOST_NO_CXX11_NOEXCEPT) && !defined(BOOST_NO_SFINAE_EXPR) && !BOOST_WORKAROUND(BOOST_GCC_VERSION, < 40800)
                                                                              ^
/Users/viboes/github/modular-boost3/boost/config/workaround.hpp:244:10: note: expanded from macro 'BOOST_WORKAROUND'
       ((symbol ## _WORKAROUND_GUARD + 0 == 0) &&     \
         ^
<scratch space>:222:1: note: expanded from here
BOOST_GCC_VERSION_WORKAROUND_GUARD
^

This is because BOOST_GCC_VERSION_WORKAROUND_GUARD is not defined and used in an expression.

The following fixes the issue.

#ifndef BOOST_GCC
#define BOOST_GCC_WORKAROUND_GUARD 1
#define BOOST_GCC_VERSION_WORKAROUND_GUARD 1
#else
#define BOOST_GCC_WORKAROUND_GUARD 0
#define BOOST_GCC_VERSION_WORKAROUND_GUARD 0
#endif
``

[eldiener]

I have created a PR for this issue.

[jzmaddock]

Since we have a PR, I'm closing this issue.