[CMake] add boost regex icu check

CMakeLists.txt

@@ -129,22 +129,29 @@ set(MAPNIK_OPTIONAL_LIBS_INCLUDE "")
 
 # needs to be before the first call of find_boost.
 if(USE_MULTITHREADED)
-    message(STATUS "multithreaded enabled")
     set(Boost_USE_MULTITHREADED ON)
     list(APPEND MAPNIK_COMPILE_DEFS MAPNIK_THREADSAFE)
 else()
     set(Boost_USE_MULTITHREADED OFF)
-    message(STATUS "multithreaded disabled")
 endif()
 
+
 find_package(PkgConfig)
 mapnik_find_threads()
 mapnik_find_package(ICU REQUIRED COMPONENTS uc i18n data)
+
 mapnik_find_package(Boost ${BOOST_MIN_VERSION} REQUIRED COMPONENTS filesystem system regex)
+list(APPEND MAPNIK_COMPILE_DEFS BOOST_REGEX_HAS_ICU)
 if(USE_BOOST_REGEX_ICU_WORKAROUND)
     message(STATUS "using boost regex workaround")
     set_property(TARGET Boost::regex PROPERTY INTERFACE_LINK_LIBRARIES)
 endif()
+include(CheckBoostRegexIcu)
+check_boost_regex()
+if(BOOST_REGEX_HAS_ICU)
+    message(STATUS "boost regex has icu support")
+    list(APPEND MAPNIK_COMPILE_DEFS BOOST_REGEX_HAS_ICU)
+endif()
 
 mapnik_find_package(Freetype REQUIRED)
 

cmake/CheckBoostRegexIcu.cmake

@@ -0,0 +1,23 @@
+include(CheckCXXSourceRuns)
+
+function(check_boost_regex)
+    set(CMAKE_REQUIRED_LIBRARIES ICU::uc ICU::data ICU::i18n Boost::headers Boost::regex)
+    check_cxx_source_runs([[
+    #include <boost/regex/icu.hpp>
+    #include <unicode/unistr.h>
+    int main()
+    {
+        U_NAMESPACE_QUALIFIER UnicodeString ustr;
+        try {
+            boost::u32regex pattern = boost::make_u32regex(ustr);
+        }
+        // an exception is fine, still indicates support is
+        // likely compiled into regex
+        catch (...) {
+            return 0;
+        }
+        return 0;
+    }
+    ]] BOOST_REGEX_HAS_ICU)
+    set(BOOST_REGEX_HAS_ICU ${BOOST_REGEX_HAS_ICU} PARENT_SCOPE)
+endfunction(check_boost_regex)