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Some systems don't have the 'dc' command installed, and this causes
configure to print a warning message unnecessarily for a standard
(non-WIP and non-pre) release of e2fsprogs.

It's easy enough to avoid this problem, so let's do it.

Addresses-Sourceforge-Bug: #1893024
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>