build(cmake): fail loudly when the readline library is found but its headers are missing

[got3nks]

Summary

Fixes the confusing Alpine build failure reported in #481:

error: 'rl_compentry_func_t' does not name a type
error: 'readline' was not declared in this scope
error: 'rl_line_buffer' was not declared in this scope
...

Root cause

cmake/FindReadline.cmake sets HAVE_LIBREADLINE from a check_function_exists(readline) that links readline / edit / editline. That succeeds not only with GNU readline but also with libedit (which exports a readline() compatibility symbol), or with a bare libreadline.so runtime and no -dev headers installed. The header probes, however, look only for <readline.h> / <readline/readline.h>.

So on a box with the library but not the matching development headers — Alpine with libedit-dev but no readline-dev is the common trigger — config.h ends up with HAVE_LIBREADLINE but no header macro. src/ExternalConnector.cpp gates its rl_* block on HAVE_LIBREADLINE alone while including the header under HAVE_READLINE_READLINE_H / HAVE_READLINE_H, so the rl_* symbols are used undeclared and the build dies deep in compilation.

Fix

Require a usable header, not just the library. When the library is found but neither header is, stop at configure time with a message naming the missing package, instead of a confusing mid-build error:

The readline library was found but its development headers were not
(neither <readline/readline.h> nor <readline.h>). Install the readline
development package and re-run cmake -- e.g. Alpine: 'apk add readline-dev';
Debian/Ubuntu: 'apt install libreadline-dev'; Fedora: 'dnf install readline-devel';
macOS (Homebrew): 'brew install readline'.

Validation

• Alpine repro (libedit-dev, no readline-dev): configure now stops with the message above; after apk add readline-dev → HAVE_LIBREADLINE=1, header found, Configuring done.
• macOS (readline header present): unaffected — no fatal, amulecmd builds.

Refs #481