Dmitry Marakasov 8b6613e592 Only check for BOOST_GCC when it's defined ...

This fixes warnings with clang:

/usr/local/include/boost/tuple/detail/tuple_basic.hpp:44:5: warning: 'BOOST_GCC' is not defined, evaluates to 0 [-Wundef]

2016-03-01 13:54:12 +03:00

..

tuple

Only check for BOOST_GCC when it's defined

2016-03-01 13:54:12 +03:00