Rely on C89 "const"

Rely on C89 "const"

The metaconfig probe for <db.h> previously relied on the d_const symbol set
by the "const" probe, so generating Configure here has been done against
metaconfig commit 1204d4627a06b11f16620188f3fa83159ed35fd9 which changes
that.

Thanks to khw++ for pointing out this oversight in my attempt last year to
make the codebase rely on C89.