[boost] fix in boost/gil/channel_algorithm.hpp for clang 3.9.1

Subject: [boost] fix in boost/gil/channel_algorithm.hpp for clang 3.9.1
From: Frédéric Bron (frederic.bron_at_[hidden])
Date: 2017-04-04 13:34:55

Hi,

When compiling with clang 3.9.1, I get an error:
/softs/osx32-clang-3.9.1/release/boost/include/boost/gil/extension/io/../../channel_algorithm.hpp:54:85:
error: non-type template argument evaluates to -1, which cannot be
narrowed to type
      'unsigned long long' [-Wc++11-narrowing]
struct unsigned_integral_max_value : public
mpl::integral_c<UnsignedIntegralChannel,-1> {};

Patch is attached.

Frédéric