Autotools: Add -fvisibility=hidden also in phpize

This builds shared extensions in php-src and through phpize with the
same visibility preset.

Author: petk

UPGRADING.INTERNALS

@@ -91,6 +91,8 @@ PHP 8.6 INTERNALS UPGRADE NOTES
 - Unix build system changes:
   . Symbol HAVE_ST_BLOCKS has been removed from php_config.h (use
     HAVE_STRUCT_STAT_ST_BLOCKS).
+  . When using phpize, extensions are now built with -fvisibility=hidden compile
+    option (when building inside php-src this was already enabled previously).
 
 ========================
 3. Module changes

scripts/phpize.m4

@@ -127,6 +127,9 @@ AS_VAR_IF([PHP_DEBUG], [yes], [
 dnl Always shared.
 PHP_BUILD_SHARED
 
+AX_CHECK_COMPILE_FLAG([-fvisibility=hidden],
+  [CFLAGS="$CFLAGS -fvisibility=hidden"])
+
 PHP_HELP_SEPARATOR([Extension:])
 PHP_CONFIGURE_PART([Configuring extension])