Required dependency on Dashel

Dashel is currently a required dependency for Aseba: https://github.com/aseba-community/aseba/blob/29b39bc39dde88f6d8f6489e7cf114d164c73e69/CMakeLists.txt#L66

However, not all Aseba products require Dashel. In particular libasebacompiler.a, libasebavm.a, libasebavmbuffer.a, and libasebavmdummycallbacks.a don't refer to Dashel.

⚠ Requiring Dashel adds an unnecessary constraint when building Asm.js and Webassembly versions of the Aseba compiler.

Of couse most Aseba products do require Dashel. Wrapping each one in a conditional DASHEL_FOUND would be fastidious and would needlessly change the indenting in every CMakeLists.txt. To avoid this, it may be simpler to only include the product subdirectories when Dashel is found. https://github.com/aseba-community/aseba/blob/29b39bc39dde88f6d8f6489e7cf114d164c73e69/CMakeLists.txt#L142-L148