Fix GH-18579: --libdir option default value

Using PHP_LIBDIR for lib64 case

UPGRADING.INTERNALS

@@ -46,6 +46,10 @@ PHP 8.5 INTERNALS UPGRADE NOTES
     without duplicate build rules.  It is up to the SAPI maintainers to ensure
     that appropriate build rules are created.
 
+- Linux build system changes
+  . libdir is properly set when --libdir (ex: /usr/lib64) and --with-libdir (ex lib64)
+    configure options are used to ${libdir}/php (ex: /usr/lib64/php)
+
 ========================
 3. Module changes
 ========================

configure.ac

@@ -1324,11 +1324,15 @@ AS_VAR_IF([program_prefix], [NONE], [program_prefix=])
 AS_VAR_IF([program_suffix], [NONE], [program_suffix=])
 
 orig_libdir=$libdir
+
+dnl First for unexpanded (default), second for expanded (from options)
 AS_CASE([$libdir],
-  ['${exec_prefix}/lib'], [libdir=$libdir/php])
+  ['${exec_prefix}/lib'], [libdir=$libdir/php],
+  [${exec_prefix}/${PHP_LIBDIR}], [libdir=$libdir/php])
 
 AS_CASE([$(eval echo $datadir)],
-  ['${prefix}/share'], [datadir=$datadir/php])
+  ['${prefix}/share'], [datadir=$datadir/php],
+  [${prefix}/share], [datadir=$datadir/php])
 
 phptempdir=$(pwd)/libs