issue #43 - bump version to 2.3.15

cmake 4.0 dropped compat support with cmake < 3.5

fixes #43

added doc for "-subst" and "-anchor-newline"

> interact -subst PATTERN::REPLACE [-anchor-newline]
> expect -anchor-newline

defined common -options (-nocase, -cstring, -lookback) as macros

updated doc for new `get -expect-buf N` command

v2.3.10: fix for test failure: tests/spawn-discard

v2.3.9: check version between client and server

fixes #25 (0.4% CPU on macOS 11.6 Big Sur)

v2.3.7: new option '-re PATTERN' for subcmd 'interact'

- default zombie idle time changed to 86400 (1d)

- -zombie-idle has a short form -z