icu4c>=75 needs c++17

Index: Makefile.am
--- Makefile.am.orig
+++ Makefile.am
@@ -10,7 +10,7 @@ liblcf_la_CPPFLAGS = \
 	-I$(srcdir)/src \
 	-I$(srcdir)/src/generated
 liblcf_la_CXXFLAGS = \
-	-std=c++11 \
+	-std=c++17 \
 	$(AM_CXXFLAGS) \
 	$(EXPAT_CFLAGS) \
 	$(ICU_CFLAGS)
@@ -203,7 +203,7 @@ test_runner_CPPFLAGS = \
 	-I$(srcdir)/src \
 	-I$(srcdir)/src/generated
 test_runner_CXXFLAGS = \
-	-std=c++11 \
+	-std=c++17 \
 	$(EXPAT_CXXFLAGS) \
 	$(ICU_CXXFLAGS)
 test_runner_LDADD = \
