[WIP] Provide common ground for all backend makefiles

[vrurg]

Provide better support for cross-backend templates.

Most crucial changes are:

• introduced term receipe macros: a class of macros responsible for generating rules.
• use_prereqs macro for replacing first target Makefile variables. Stores its content in @prereqs@ variable in enclosing receipe macro context.
• the above required NQP::Config::set method to support context name in in_ctx named parameter.

Related to perl6/nqp#580 and rakudo/rakudo#3194