Update libtool to 2.5.4

The libtool 1.5.26 is bundled with PHP since the very early days of the
Autotools build system to ease the building process and avoid additional
dependency on the system Libtool. This updates the bundled libtool to
the current 2.5.4 version.

Fixes and implementations:

- Fixed race conditions when building PHP in parallel ("cannot create
  .libs" warnings).
- Implements request #70374 (Update libtool.m4)

Changes:
- On macOS linker now uses -undefined dynamic_lookup flag for shared
  extensions and shared embed SAPI (libphp) instead of older
  '-undefined suppress -flat_namespace' combination.

Steps done:

```
git clone https://git.savannah.gnu.org/git/libtool.git
cd libtool
git checkout v2.5.4
./bootstrap

cp build-aux/ltmain.sh \
  m4/libtool.m4 \
  m4/lt\~obsolete.m4 \
  m4/ltoptions.m4 \
  m4/ltsugar.m4 \
  m4/ltversion.m4 \
  ../php-src/build/

cd ../php-src
sed -e 's/^#[ ]*serial .*lt.*/dnl &/g' < build/libtool.m4 \
  > build/libtool.m4.tmp && mv build/libtool.m4.tmp build/libtool.m4
```

Author: petk

Zend/Zend.m4

@@ -118,7 +118,7 @@ dnl Ugly hack to check if dlsym() requires a leading underscore in symbol name.
 dnl
 AC_DEFUN([ZEND_DLSYM_CHECK], [dnl
 AC_MSG_CHECKING([whether dlsym() requires a leading underscore in symbol names])
-_LT_AC_TRY_DLOPEN_SELF([AC_MSG_RESULT([no])], [
+_LT_TRY_DLOPEN_SELF([AC_MSG_RESULT([no])], [
   AC_MSG_RESULT([yes])
   AC_DEFINE([DLSYM_NEEDS_UNDERSCORE], [1],
     [Define to 1 if 'dlsym()' requires a leading underscore in symbol names.])