Channel tests failing for variant=release with GCC

Following addition of `variant=debug,release` to Travis CI in #50, the GCC 5.4.1 build job is failing due to failing channel tests, see https://travis-ci.org/boostorg/gil/jobs/355009262
```
testing.capture-output bin.v2/libs/gil/test/channel.test/gcc-5.4.1/release/channel.run
====== BEGIN OUTPUT ======
std::exception
EXIT STATUS: 1
====== END OUTPUT ======
```
Interestingly, the same tests built with clang 4.2.1 are passing without any failures, see https://travis-ci.org/boostorg/gil/jobs/355009263

I confirmed this issue locally, on Ubuntu with GCC 5.4.0
```
b2 variant=debug,release cxxflags="-Wno-unused-local-typedefs" test
...
testing.capture-output ../../bin.v2/libs/gil/test/channel.test/gcc-5.4.0/debug/channel.run
**passed** ../../bin.v2/libs/gil/test/channel.test/gcc-5.4.0/debug/channel.test
...
testing.capture-output ../../bin.v2/libs/gil/test/channel.test/gcc-5.4.0/release/channel.run
====== BEGIN OUTPUT ======
std::exception

EXIT STATUS: 1
====== END OUTPUT ======
```

## References

Yet another issue revealed by `variant=release` build configuration.
Previous one was the image checksum tests, see #49