Sylpheed-Claws compile error on Slackware-current
 

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You could do a "locate libdb" to see where it really is. Did you installed libdb as a Slackware package, or did you compile it yourself?

You may need to set LD_CONFIG_PATH or LD_LIBRARY_PATH:
export LD_LIBRARY_PATH=/usr/local/lib
export LD_CONFIG_PATH=/usr/local/lib

It's just a guess, if that doesn't work, post some more information. Good luck!

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Sylpheed uses version 4.2 instead of 4.4, so you may want to install the older version.

Also, you could make a simbolic link, libdb.4.4.la -> libdb.4.2.la. It's dirty, but it worked for me most of the times.