Brew doctor: dyld: Library not loaded & Error: No available formula for zlib

For me it works with:

sudo ln -s /usr/local/Cellar/libtool/2.4.2/lib/libltdl.7.dylib /usr/local/lib/libltdl.7.dylib

Try to re-install libtool by:

brew reinstall libtool --universal && brew unlink libtool && brew link libtool

If that doesn't help, try removing libtool completely, and then retry the steps above:

brew uninstall libtool

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If it still doesn't work after trying the steps above, check to see if you have the DYLD_FALLBACK_LIBRARY_PATH variable defined somewhere on the system (e.g. ~/.profile) and try unsetting it before trying the steps above again.

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Other commands which could be useful for identifying the issue:

$ libtool --version
$ whereis libtool
$ set | grep DYLD_FALLBACK_LIBRARY_PATH
$ echo $DYLD_FALLBACK_LIBRARY_PATH