php7: use TARGET_CXXFLAGS instead of TARGET_CPPFLAGS for C++ option

This fixes a stupid typo from last commit which lead to the following
compiler warning:

cc1: warning: command line option '-std=c++11' is valid for C++/ObjC++ but not for C

Signed-off-by: Michael Heimpold <mhei@heimpold.de>

lang/php7/Makefile

@@ -7,7 +7,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=php
 PKG_VERSION:=7.1.4
-PKG_RELEASE:=4
+PKG_RELEASE:=5
 
 PKG_MAINTAINER:=Michael Heimpold <mhei@heimpold.de>
 
@@ -245,7 +245,7 @@ endif
 
 ifneq ($(SDK)$(CONFIG_PACKAGE_php7-mod-intl),)
   CONFIGURE_ARGS+= --enable-intl=shared
-  TARGET_CPPFLAGS+= -std=c++0x
+  TARGET_CXXFLAGS+= -std=c++0x
 else
   CONFIGURE_ARGS+= --disable-intl
 endif