configure: clarify --enable-debug option

- Change --enable-debug help text to say it is for developing curl.

- Warn when --enable-debug is used.

This change copies the help text and warning from cmake ENABLE_DEBUG.
Also, it shortens the "for debugging curl itself" to just say "for
debugging curl".

Now it looks like this:

  --enable-debug          Enable curl debug features (for developing
                          curl)
  --disable-debug         Disable curl debug features

Ref: https://github.com/curl/curl/issues/22481#issuecomment-5207220359

Reported-by: Viktor Szakats

Closes https://github.com/curl/curl/pull/22512

CMakeLists.txt

@@ -257,7 +257,7 @@ if(CYGWIN OR CMAKE_SYSTEM_NAME STREQUAL "Linux" OR CMAKE_SYSTEM_NAME STREQUAL "G
   list(APPEND CMAKE_REQUIRED_DEFINITIONS "-D_GNU_SOURCE")  # Apply to all feature checks
 endif()
 
-option(ENABLE_DEBUG "Enable curl debug features (for developing curl itself)" OFF)
+option(ENABLE_DEBUG "Enable curl debug features (for developing curl)" OFF)
 if(ENABLE_DEBUG)
   message(WARNING "This curl build is Debug-enabled and insecure, do not use in production.")
 endif()

configure.ac

@@ -5619,6 +5619,10 @@ AC_MSG_NOTICE([Configured to build curl/libcurl:
   Features:         ${SUPPORT_FEATURES}
 ])
 
+if test "$want_debug" = "yes"; then
+  AC_MSG_WARN([This curl build is Debug-enabled and insecure, do not use in production.])
+fi
+
 if test -n "$experimental"; then
   for a in $experimental; do
     AC_MSG_WARN([$a is enabled but marked EXPERIMENTAL. Use with caution!])

docs/INSTALL.md

@@ -146,8 +146,8 @@ Building statically is not for the faint of heart.
 
 ## Debug
 
-If you are a curl developer and use gcc, you might want to enable more debug
-options with the `--enable-debug` option.
+If you are a curl developer and use gcc, you might want to enable curl debug
+features (for developing curl) with the `--enable-debug` option.
 
 curl can be built to use a whole range of libraries to provide various useful
 services, and configure tries to auto-detect a decent default. If you want to

m4/curl-confopts.m4

@@ -113,8 +113,8 @@ AC_DEFUN([CURL_CHECK_OPTION_DEBUG], [
   AC_MSG_CHECKING([whether to enable debug build options])
   OPT_DEBUG_BUILD="default"
   AC_ARG_ENABLE(debug,
-AS_HELP_STRING([--enable-debug],[Enable debug build options])
-AS_HELP_STRING([--disable-debug],[Disable debug build options]),
+AS_HELP_STRING([--enable-debug],[Enable curl debug features (for developing curl)])
+AS_HELP_STRING([--disable-debug],[Disable curl debug features]),
   OPT_DEBUG_BUILD=$enableval)
   case "$OPT_DEBUG_BUILD" in
     no)