Description:
------------
Compilation of ext-intl fails with libicu 59+ and GCC 8.

Caused by https://github.com/php/php-src/commit/5dd1ef90caec3021e6ce55c8554e695edf641eaf.

Test script:
---------------
Dockerfile:
====================
FROM debian:experimental

RUN apt-get update && apt-get install -y --no-install-recommends make autoconf gcc-8 g++-8 libicu-dev bison ca-certificates curl
RUN mkdir /php && cd /php
RUN curl -Lq https://github.com/php/php-src/archive/5dd1ef90caec3021e6ce55c8554e695edf641eaf.tar.gz | tar -xzf - --strip-components=1
RUN ./buildconf && CC=gcc-8 CXX=g++-8 ./configure --disable-all --enable-intl && make
====================

run as:
docker build -t php - < Dockerfile

Expected result:
----------------
no error

Actual result:
--------------
/bin/bash //libtool --silent --preserve-dup-deps --mode=compile g++-8 -I/usr/include  -Wno-write-strings -D__STDC_LIMIT_MACROS -DZEND_ENABLE_STATIC_TSRMLS_CACHE=1 -std=c++11  -DU_USING_ICU_NAMESPACE=1 -std=c++11 -stdlib=libc++ -Iext/intl/ -I//ext/intl/ -DPHP_ATOM_INC -I//include -I//main -I/ -I/ext/date/lib -I//ext/date/lib -I//TSRM -I//Zend    -g -O2  -c //ext/intl/intl_convertcpp.cpp -o ext/intl/intl_convertcpp.lo 
g++-8: error: unrecognized command line option '-stdlib=libc++'
make: *** [Makefile:632: ext/intl/intl_convertcpp.lo] Error 1