How to debug brew receipt setup

brew debugging:

Error: You must brew link pkg-config’ before lftp can be installed

brew link --overwrite --dry-run pkg-config
brew link --overwrite xz

another debugging example

$brew install nmap
Warning: nmap-6.40 already installed, it's just not linked
$brew link nmap
Linking /usr/local/Cellar/nmap/6.40... Warning: Could not link nmap.
Unlinking...
$brew link --overwrite nmap