You can see that these are automatically set by Homebrew to include dependencies' private prefixes without needing to add anything:

> PKG_CONFIG_PATH=/usr/local/opt/libxml2/lib/pkgconfig:/usr/local/opt/libtasn1/lib/pkgconfig:/usr/local/opt/openssl/lib/pkgconfig:/usr/local/opt/nettle/lib/pkgconfig:/usr/local/opt/gnutls/lib/pkgconfig:/usr/local/opt/libxmlsec1/lib/pkgconfig:/usr/local/opt/libxslt/lib/pkgconfig:/usr/local/opt/ktoblzcheck/lib/pkgconfig
> 
> PATH=/usr/local/Cellar/python/2.7.6/Frameworks/Python.framework/Versions/2.7/Resources/Python.app/Contents/MacOS:/usr/local/Library/ENV/3.1.4:/usr/local/opt/pkg-config/bin:/usr/local/opt/libxml2/bin:/usr/local/opt/libtasn1/bin:/usr/local/opt/openssl/bin:/usr/local/opt/nettle/bin:/usr/local/opt/gnutls/bin:/usr/local/opt/libgpg-error/bin:/usr/local/opt/libgcrypt/bin:/usr/local/opt/libxmlsec1/bin:/usr/local/opt/libxslt/bin:/usr/local/opt/gettext/bin:/usr/local/opt/ktoblzcheck/bin:/usr/bin:/bin:/usr/sbin:/sbin