Update ICU testing and usage.

CMakeLists.txt

@@ -13,11 +13,57 @@ add_library(Boost::regex ALIAS boost_regex)
 
 target_include_directories(boost_regex INTERFACE include)
 
-target_link_libraries(boost_regex
-  INTERFACE
-    Boost::config
-    Boost::throw_exception
-    Boost::predef
-    Boost::assert
-)
+option(BOOST_REGEX_STANDALONE "Boost.Regex: Enable Standalone Mode (i.e. no Boost dependencies)")
+
+if(NOT BOOST_REGEX_STANDALONE)
+
+   target_link_libraries(boost_regex
+     INTERFACE
+       Boost::config
+       Boost::throw_exception
+       Boost::predef
+       Boost::assert
+   )
+
+else()
+
+   target_compile_definitions(boost_regex
+     INTERFACE BOOST_REGEX_STANDALONE
+   )
+
+endif()
+
+find_package(ICU COMPONENTS data i18n uc QUIET)
+#option(BOOST_REGEX_ENABLE_ICU "Boost.Regex: enable ICU support" ${ICU_FOUND})
+
+if(ICU_FOUND)
+
+   add_library(boost_regex_icu INTERFACE)
+   add_library(Boost::regex_icu ALIAS boost_regex_icu)
+
+   target_include_directories(boost_regex_icu INTERFACE include)
+
+   if(NOT BOOST_REGEX_STANDALONE)
+
+      target_link_libraries(boost_regex_icu
+        INTERFACE
+          Boost::config
+          Boost::throw_exception
+          Boost::predef
+          Boost::assert
+      )
+
+   else()
+
+      target_compile_definitions(boost_regex_icu
+        INTERFACE BOOST_REGEX_STANDALONE
+      )
+
+   endif()
+
+   find_package(ICU COMPONENTS data i18n uc REQUIRED)
+
+   target_link_libraries(boost_regex_icu INTERFACE ICU::data ICU::i18n ICU::uc)
+
+endif()