Add BUILD_REQUIRES and {PROFILE,SETTINGS,OPTIONS}_{BUILD,HOST}

[puetzk]

This was previously rejected in #67 (comment), but the rationale there is no longer true. conanfile.txt does support [build_requires] since conan 1.1.0, and they are not interchangeable when cross-compiling.

We have a number of consumers that use conan_cmake_run(REQUIRES ...) because they have a lot of CMake build options that affect which requirements to pull in. While it would be possible to define a conanfile.py with corresponding options, this is rather awkward compared to just being able to put conditional logic in the CMakeLists.txt deciding what they want. It would be nice if cmake-conan had at least the bare minimum to put packages in [build_requires], and to pass --{profile,options,settings}:{host,build}

TODO: how should this interact with PROFILE_AUTO? Most such should presumably go to SETTINGS_HOST, but maybe some of the CMAKE_HOST_* variables could help to fill in SETTINGS_BUILD (note mismatch of terminology - CMake calls the system CMake is running on CMAKE_HOST_*, which is what conan calls "build"

[puetzk]

The requested options seem to have been added in #250 and #310, thanks!