add option to conditionally build manual selector node

[alsora]

Currently, the code looks whether ncurses is found and if that's the case, it tries to build the manual selector node.
This PR adds an explicit option to enable/disable building this node.

Besides giving more control on the dependencies, this simplifies cross-compilation procedures for example when building this package in conan (see here https://github.com/conan-io/conan-center-index/blob/master/recipes/behaviortree.cpp/all/conanfile.py)

Conan requires that all dependencies that will be "searched" (via find_package) are listed as requirements in the conanfile and it does not play well with optional dependencies that are not controlled via CMake options.
In particular, without my change, the code implicitly required the cross-compilation of ncurses.

• in order to specify ncurses as a dependency in the conanfile it is required to cross-compile it.
• if a user didn't specify ncurses as a dependency, find_package(Curses QUIET) could still find an x86_64 version if this was present in the OS (e.g. ubuntu desktop) thus linking it with the library and obviously failing the compilation as the library format was incompatible.

Signed-off-by: Alberto Soragna alberto.soragna@gmail.com

[facontidavide]

I like the idea, but this PR breaks compilation on those platforms that don't have ncurse installed.
As you can see, the Window CI is failing

[facontidavide]

thanks