Fix bug #51558: shared readline build fails (php#15242)

The 'rl_pending_input' is a variable in Readline library and checking it
with PHP_CHECK_LIBRARY wouldn't find it on some systems.

Library check works on most systems but not on the mentioned AIX in the
bug as it exports variables and functions differently whereas the linker
couldn't resolve the variable as a function.

This should fix the build on systems where this caused issues, such as
AIX.

The <readline/readline.h> is not self-contained header and needs to also
have <stdio.h> included before to have FILE type available. This fixes
the issue on unpatched default readline installations, such as macOS.

Checking this variable ensures that the found library is the correct
library and also that it is of minimum version needed by current PHP
code (https://bugs.php.net/48608).

The library check:

```c
| char rl_pending_input ();
| int main (void) {
|     return rl_pending_input ();
| }
```

The declaration check:

```c
| #include <stdio.h>
| #include <readline/readline.h>
| int main (void) {
| #ifndef rl_pending_input
| #ifdef __cplusplus
|     (void) rl_pending_input;
| #else
|     (void) rl_pending_input;
| #endif
| #endif
| ;
|     return 0;
| }
```

Closes https://bugs.php.net/51558

Closes phpGH-19259.

Author: petk

NEWS

@@ -35,6 +35,10 @@ PHP                                                                        NEWS
   . Fixed dangling pointer access on _pdo_pgsql_trim_message helper.
     (dixyes)
 
+- Readline:
+  . Fixed bug GH-19250 and bug #51360 (Invalid conftest for rl_pending_input).
+    (petk, nielsdos)
+
 - SOAP:
   . Fixed bug GH-18640 (heap-use-after-free ext/soap/php_encoding.c:299:32
     in soap_check_zval_ref). (nielsdos)

ext/readline/config.m4

@@ -47,13 +47,6 @@ if test "$PHP_READLINE" && test "$PHP_READLINE" != "no"; then
     -L$READLINE_DIR/$PHP_LIBDIR $PHP_READLINE_LIBS
   ])
 
-  PHP_CHECK_LIBRARY(readline, rl_pending_input,
-  [], [
-    AC_MSG_ERROR([invalid readline installation detected. Try --with-libedit instead.])
-  ], [
-    -L$READLINE_DIR/$PHP_LIBDIR $PHP_READLINE_LIBS
-  ])
-
   PHP_CHECK_LIBRARY(readline, rl_callback_read_char,
   [
     AC_DEFINE(HAVE_RL_CALLBACK_READ_CHAR, 1, [ ])
@@ -75,6 +68,26 @@ if test "$PHP_READLINE" && test "$PHP_READLINE" != "no"; then
     -L$READLINE_DIR/$PHP_LIBDIR $PHP_READLINE_LIBS
   ])
 
+  CFLAGS_SAVE=$CFLAGS
+  LDFLAGS_SAVE=$LDFLAGS
+  LIBS_SAVE=$LIBS
+  CFLAGS="$CFLAGS $INCLUDES"
+  LDFLAGS="$LDFLAGS -L$READLINE_DIR/$PHP_LIBDIR"
+  LIBS="$LIBS -lreadline"
+
+  dnl Sanity and minimum version check if readline library has variable
+  dnl rl_pending_input.
+  AC_CHECK_DECL([rl_pending_input],, [AC_MSG_FAILURE([
+      Invalid readline installation detected. Try --with-libedit instead.
+    ])], [
+      #include <stdio.h>
+      #include <readline/readline.h>
+    ])
+
+  CFLAGS=$CFLAGS_SAVE
+  LDFLAGS=$LDFLAGS_SAVE
+  LIBS=$LIBS_SAVE
+
   AC_DEFINE(HAVE_HISTORY_LIST, 1, [ ])
   AC_DEFINE(HAVE_LIBREADLINE, 1, [ ])