By default now it's assumed that if $build_style is not set, the template
uses 'custom-install' build, .e.g do_{build,configure,install}.
If it's set, a helper with the same name with .sh extension will be sourced
to set do_{build,configure,install} phases.
The exception is "meta-template" which currently it must be set via
build_style, probably will change in the future.
