Link coroutine_exe against CONAN_LIBS

[sturmf]

This allows boost to be built on Ubuntu 18.04 with older CMake that does not know about newer Boost versions and wouldn't add the boost_context lib to coroutine_exe in the static linking case.

See conan-io#3159 for a lengthy discussion of the issue.

Specify library name and version: boost/1.70

• [x ] I've read the guidelines for contributing.
• [x ] I've followed the PEP8 style guides for Python code in the recipes.
• [x ] I've used the latest Conan client version.
• I've tried at least one configuration locally with the
  conan-center hook activated.