build: add curl-lint/lint targets, CURL_LINT cmake option

To run checksrc and spacecheck on the source tree. Also for cmake
to sync up with autotools' `checksrc` target.

- cmake: `curl-lint`
  With `-DCURL_LINT=ON`, checks run automatically for all targets.
- autotools: `lint`

Closes #20175

docs/INSTALL-CMAKE.md

@@ -241,6 +241,7 @@ target_link_libraries(my_target PRIVATE CURL::libcurl)
 - `CURL_LIBCURL_SOVERSION`:                 Enable libcurl SOVERSION. Default: `ON` for supported platforms
 - `CURL_LIBCURL_VERSIONED_SYMBOLS`:         Enable libcurl versioned symbols. Default: `OFF`
 - `CURL_LIBCURL_VERSIONED_SYMBOLS_PREFIX`:  Override default versioned symbol prefix. Default: `<TLS-BACKEND>_` or `MULTISSL_`
+- `CURL_LINT`:                              Run lint checks while building. Default: `OFF`
 - `CURL_LTO`:                               Enable compiler Link Time Optimizations. Default: `OFF`
 - `CURL_STATIC_CRT`:                        Build libcurl with static CRT with MSVC (`/MT`) (requires UCRT, static libcurl or no curl executable). Default: `OFF`
 - `CURL_TARGET_WINDOWS_VERSION`:            Minimum target Windows version as hex string.
@@ -544,6 +545,7 @@ Note: These variables are internal and subject to change.
 - `curl-completion-zsh`:    Build shell completions for zsh (built by default if enabled)
 - `curl-ca-bundle`:         Build the CA bundle via `scripts/mk-ca-bundle.pl`
 - `curl-ca-firefox`:        Build the CA bundle via `scripts/firefox-db2pem.sh`
+- `curl-lint`:              Run lint checks.
 - `curl-listcats`:          Generate help category constants for `src/tool_help.h` from documentation.
 - `curl-listhelp`:          Generate `src/tool_listhelp.c` from documentation.
 - `curl-optiontable`:       Generate `lib/easyoptions.c` from documentation.