Feature: Enable access to build.environment object methods

[RobinMcCorkell]

Certain methods, in particular get_exe_suffix(), get_shared_lib_suffix() and get_*dir(), can be very useful to use in meson.build. For example, when using install_subdirs to install a header hierarchy it would be ideal to not hardcode 'include' as the destination, instead using something like environment.get_includedir(). I don't know if the meson object is the best place for these methods, or if a new environment object should be created.