Fix boost::regex ICU support

configure.ac

@@ -152,7 +152,7 @@ PKG_CHECK_MODULES([ICU], [icu-i18n icu-uc], [
 	old_LIBS="$LIBS"
 	AC_SUBST(ICU_CFLAGS)
 	AC_SUBST(ICU_LIBS)
-	CPPFLAGS="$CPPFLAGS $ICU_CFLAGS -DU_USING_ICU_NAMESPACE=0"
+	CPPFLAGS="$CPPFLAGS $ICU_CFLAGS"
 	LIBS="$LIBS $ICU_LIBS"
 	AC_MSG_CHECKING([whether boost.regex was compiled with ICU support])
 	AC_LINK_IFELSE([AC_LANG_PROGRAM([

src/regex_filter.h

@@ -93,7 +93,7 @@ inline Regex make(StringT &&s,
 template <typename CharT>
 inline bool search(const std::basic_string<CharT> &s,
                    const Regex &rx,
-                   [[maybe_unused]] bool ignore_diacritics)
+                   bool ignore_diacritics)
 {
 	try {
 #ifdef BOOST_REGEX_ICU