Mojave + Nokogiri instalation.

nokogiri_install.txt

# The main idea to install three libraries.
# These libraries can't be symlinked because they are macOs-provided
# > Refusing to link macOS-provided software: libxml2
# So, You need to specify lib and include directories.

brew install libxml2
brew install libxslt
brew install libiconv

gem install nokogiri -v '1.6.2.1' -- \
    --use-system-libraries \ 
    --with-xml2-lib=/usr/local/Cellar/libxml2/2.9.7/lib \
    --with-xml2-include=/usr/local/Cellar/libxml2/2.9.7/include/libxml2 \
    --with-xslt-lib=/usr/local/Cellar/libxslt/1.1.32/lib \
    --with-xslt-include=/usr/local/Cellar/libxslt/1.1.32/include/libxslt \
    --with-iconv-lib=/usr/local/Cellar/libiconv/1.15/lib \
    --with-iconv-include=/usr/local/Cellar/libiconv/1.15/include