Prefer to find icu via pkg-config instead of icu-config. Honor the CXXFLAGS that
are indicated by either of those tools. Work around new ICU requirement that a
namespace be used by specifying the flag that overrides that requirement.

Includes relevant parts of the following upstream commits:

Fix #73956 Link use CC instead of CXX
https://github.com/php/php-src/commit/6af22d0d3ed224d89be1d1e9e53ff7e2e2e5439d

Fixed bug #76153 Intl compilation fails with icu4c 61.1
https://github.com/php/php-src/commit/710284cbc4a54cac0a9ec4ea29a7486e0d99a33b

Fix clang build, ref buf #76153
https://github.com/php/php-src/commit/8b104d789317d96f6d3e23e635f0ca288c0a23ee

Fix shared ext/intl compilation
https://github.com/php/php-src/commit/2bd299f7318492fd7e5cafffa562d76ba60e69d4

Rename var
https://github.com/php/php-src/commit/ed5aabe8b78d8487ffc6091e0670753d59bb7f5a

Group common flags
https://github.com/php/php-src/commit/09d7ffabcd99fe9b99c4206f321fecc506635072
--- ext/intl/config.m4.orig	2019-01-09 03:54:13.000000000 -0600
+++ ext/intl/config.m4	2019-10-22 17:47:47.000000000 -0500
@@ -9,6 +9,7 @@
   PHP_SETUP_ICU(INTL_SHARED_LIBADD)
   PHP_SUBST(INTL_SHARED_LIBADD)
   PHP_REQUIRE_CXX()
+  INTL_COMMON_FLAGS="$ICU_INCS -Wno-write-strings"
   if test "$icu_version" -ge "4002"; then
     icu_spoof_src=" spoofchecker/spoofchecker_class.c \
     spoofchecker/spoofchecker.c\
@@ -20,7 +21,6 @@
   PHP_NEW_EXTENSION(intl, php_intl.c \
     intl_error.c \
     intl_convert.c \
-	intl_convertcpp.cpp \
     collator/collator.c \
     collator/collator_class.c \
     collator/collator_sort.c \
@@ -32,8 +32,6 @@
     collator/collator_is_numeric.c \
     collator/collator_error.c \
     common/common_error.c \
-	common/common_enum.cpp \
-	common/common_date.cpp \
     converter/converter.c \
     formatter/formatter.c \
     formatter/formatter_main.c \
@@ -53,17 +51,12 @@
     dateformat/dateformat_attr.c \
     dateformat/dateformat_data.c \
     dateformat/dateformat_format.c \
-    dateformat/dateformat_format_object.cpp \
     dateformat/dateformat_parse.c \
-    dateformat/dateformat_create.cpp \
-    dateformat/dateformat_attrcpp.cpp \
-    dateformat/dateformat_helpers.cpp \
     msgformat/msgformat.c \
     msgformat/msgformat_attr.c \
     msgformat/msgformat_class.c \
     msgformat/msgformat_data.c  \
     msgformat/msgformat_format.c \
-    msgformat/msgformat_helpers.cpp \
     msgformat/msgformat_parse.c \
     grapheme/grapheme_string.c \
     grapheme/grapheme_util.c \
@@ -73,6 +66,17 @@
     transliterator/transliterator.c \
     transliterator/transliterator_class.c \
     transliterator/transliterator_methods.c \
+    idn/idn.c \
+    $icu_spoof_src, $ext_shared,,$INTL_COMMON_FLAGS,cxx)
+
+  PHP_INTL_CXX_SOURCES="intl_convertcpp.cpp \
+    common/common_enum.cpp \
+    common/common_date.cpp \
+    dateformat/dateformat_format_object.cpp \
+    dateformat/dateformat_create.cpp \
+    dateformat/dateformat_attrcpp.cpp \
+    dateformat/dateformat_helpers.cpp \
+    msgformat/msgformat_helpers.cpp \
     timezone/timezone_class.cpp \
     timezone/timezone_methods.cpp \
     calendar/calendar_class.cpp \
@@ -83,9 +87,14 @@
     breakiterator/breakiterator_methods.cpp \
     breakiterator/rulebasedbreakiterator_methods.cpp \
     breakiterator/codepointiterator_internal.cpp \
-    breakiterator/codepointiterator_methods.cpp \
-    idn/idn.c \
-    $icu_spoof_src, $ext_shared,,$ICU_INCS -Wno-write-strings)
+    breakiterator/codepointiterator_methods.cpp"
+  PHP_INTL_CXX_FLAGS="$INTL_COMMON_FLAGS $ICU_CXXFLAGS"
+  if test "$ext_shared" = "no"; then
+    PHP_ADD_SOURCES(PHP_EXT_DIR(intl), $PHP_INTL_CXX_SOURCES, $PHP_INTL_CXX_FLAGS)
+  else
+    PHP_ADD_SOURCES_X(PHP_EXT_DIR(intl), $PHP_INTL_CXX_SOURCES, $PHP_INTL_CXX_FLAGS, shared_objects_intl, yes)
+  fi
+
   PHP_ADD_BUILD_DIR($ext_builddir/collator)
   PHP_ADD_BUILD_DIR($ext_builddir/converter)
   PHP_ADD_BUILD_DIR($ext_builddir/common)