Autotools: Enhance phpdbg readline enabling

PHP_READLINE, PHP_LIBEDIT, and PHP_READLINE_LIBS variables are not
available in sapi before the extensions arguments get processed by the
configure script. But the Autoconf raw default variables with_readline
and with_libedit are. This checks if readline support can be enabled in
phpdbg

This is partial fix towards the more proper phpGH-13184 or similar in the
future to make phpdbg readline integration easier to use.

Previous check silently enabled the phpdbg readline integration when
readline is built as shared and failed on the make step:

    undefined reference to 'readline'
    undefined reference to 'add_history'

These cases enable it:
    ./configure --enable-phpdbg-readline --with-libedit
    ./configure --enable-phpdbg-readline --with-readline
These cases don't enable it:
    ./configure --enable-phpdbg-readline
    ./configure --enable-phpdbg-readline --with-libedit=shared
    ./configure --enable-phpdbg-readline --with-readline=shared

sapi/phpdbg/config.m4

@@ -21,26 +21,30 @@ PHP_ARG_ENABLE([phpdbg-readline],
   [no])
 
 if test "$PHP_PHPDBG" != "no"; then
+  AS_VAR_IF([PHP_PHPDBG_READLINE], [yes], [
+    AC_MSG_CHECKING([if readline can be enabled in phpdbg])
+    phpdbg_readline="no (readline extension is not enabled)"
+    AS_IF(m4_normalize([
+      (test -n "$with_readline" && test "x$with_readline" != xno) ||
+      (test -n "$with_libedit" && test "x$with_libedit" != xno)
+    ]), [phpdbg_readline=yes])
+    AS_CASE([$with_readline], [*shared*],
+      [phpdbg_readline="no (shared readline extension not supported)"])
+    AS_CASE([$with_libedit], [*shared*],
+      [phpdbg_readline="no (shared readline extension not supported)"])
+    AS_VAR_IF([phpdbg_readline], [yes], [
+      AC_DEFINE([HAVE_PHPDBG_READLINE], [1],
+        [Define to 1 if the phpdbg SAPI has libedit/readline integration.])
+    ])
+    AC_MSG_RESULT([$phpdbg_readline])
+  ])
+
   AC_HEADER_TIOCGWINSZ
 
   PHP_PHPDBG_CFLAGS="-DZEND_ENABLE_STATIC_TSRMLS_CACHE=1"
   AS_VAR_IF([PHP_PHPDBG_DEBUG], [no],,
     [AS_VAR_APPEND([PHP_PHPDBG_CFLAGS], [" -DPHPDBG_DEBUG=1"])])
 
-  AC_MSG_CHECKING([for phpdbg and readline integration])
-  if test "$PHP_PHPDBG_READLINE" = "yes"; then
-    if test "$PHP_READLINE" != "no" || test "$PHP_LIBEDIT" != "no"; then
-      AC_DEFINE([HAVE_PHPDBG_READLINE], [1],
-        [Define to 1 if the phpdbg SAPI has libedit/readline integration.])
-      PHPDBG_EXTRA_LIBS="$PHP_READLINE_LIBS"
-      AC_MSG_RESULT([ok])
-    else
-      AC_MSG_RESULT([readline is not available])
-    fi
-  else
-    AC_MSG_RESULT([disabled])
-  fi
-
   AH_TEMPLATE([HAVE_USERFAULTFD_WRITEFAULT],
     [Define to 1 if faulting on write-protected memory support can be compiled
     for userfaultfd.])