HOMEBREW_PATCHELF_RB is enabled on 2 cases.
1) `HOMEBREW_PATCHELF_RB` is set
2) `HOMEBREW_DEVELOPER` is set , and `HOMEBREW_NO_PATCHELF_RB` is not set.
use `HOMEBREW_NO_PATCHELF_RB` to turn it off for devs.
Note: When HOMEBREW_PATCHELF_RB and HOMEBREW_NO_PATCHELF_RB both are
present, it is on
Having HOMEBREW_PATCHELF_RB set in the ENV,
will conditionally install patchelf.rb gem,
use patchelf.rb in the above mentioned methods.
The installed vendored gems are listed in .gitignore
to maintain a clean state.
