Fixes a regression introduced by Homebrew/homebrew#47950 in 9e3ee3e causing build
failures with the error
cabal: The file does not exist ''.
This will occur whenever the optional :flags key isn't in the options
hash passed to install_cabal_package.
ClosesHomebrew/homebrew#49425.
Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
This is the (partially) documented public API for Homebrew.
The main class you should look at is the {Formula} class (and classes linked from there). That's the class that's used to create Homebrew formulae (i.e. package descriptions). Assume anything else you stumble upon is private.