Fish shell installation using homebrew - iconv libiconv error
In the log that you posted above the following three lines suggest that you have MacPorts installed.

checking for /opt/local/include include directory... yes
checking for /opt/local/lib library directory... yes
checking for /opt/local/bin command directory... yes


It also appears that brew is checking there before checking in its regular file locations (/usr/local, etc). Something under /opt might be confusing brew and preventing it from compiling fish. 

This can be mended by following MacPorts uninstall guide: guide.macports.org/#installing.macports.uninstalling