Use ICU's CXXFLAGS when using pkg-config
https://github.com/php/php-src/commit/8daf96cef35c114b9e9cbcc47fc8602be74754d3
--- a/acinclude.m4	2024-05-19 15:08:42.000000000 -0500
+++ b/acinclude.m4	2024-05-19 15:08:42.000000000 -0500
@@ -2236,7 +2236,8 @@
 
       ICU_LIBS=`$PKG_CONFIG --libs icu-uc icu-io icu-i18n`
       ICU_INCS=`$PKG_CONFIG --cflags-only-I icu-uc icu-io icu-i18n`
-      ICU_CXXFLAGS="-DU_USING_ICU_NAMESPACE=1"
+      ICU_CXXFLAGS=`$PKG_CONFIG --variable=CXXFLAGS icu-uc`
+      ICU_CXXFLAGS="$ICU_CXXFLAGS -DU_USING_ICU_NAMESPACE=1"
 
       AC_MSG_RESULT([found $ICU_VERSION])
 
