Add a detailed troubleshooting message for a likely avr-libc error

In avr-libc 1.8 the FUSES helper macro does not result in adding attribute
"used". The section can be dropped due to linker optimizations.  This appears
to be fixed in avr-libc as of at least svn r2297. Inform the user of a
workaround.

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Merge pull request #1 from prutschman/master

Inform the user of a workaround for an avr-libc 1.8 bug that can cause the .fuse section to be lost during linking.