Switch Automake build system to C++17.

Makefile.in

@@ -136,10 +136,11 @@ noinst_PROGRAMS = $(am__EXEEXT_1) $(am__EXEEXT_2) \
 @BUILD_WITH_WCHAR_T_SUPPORT_TRUE@am__append_29 = unit_testsU
 subdir = .
 ACLOCAL_M4 = $(top_srcdir)/aclocal.m4
-am__aclocal_m4_deps = $(top_srcdir)/m4/libtool.m4 \
-	$(top_srcdir)/m4/ltoptions.m4 $(top_srcdir)/m4/ltsugar.m4 \
-	$(top_srcdir)/m4/ltversion.m4 $(top_srcdir)/m4/lt~obsolete.m4 \
-	$(top_srcdir)/acinclude.m4 $(top_srcdir)/m4/ax_c_ifdef.m4 \
+am__aclocal_m4_deps = $(top_srcdir)/m4/ax_cxx_compile_stdcxx.m4 \
+	$(top_srcdir)/m4/libtool.m4 $(top_srcdir)/m4/ltoptions.m4 \
+	$(top_srcdir)/m4/ltsugar.m4 $(top_srcdir)/m4/ltversion.m4 \
+	$(top_srcdir)/m4/lt~obsolete.m4 $(top_srcdir)/acinclude.m4 \
+	$(top_srcdir)/m4/ax_c_ifdef.m4 \
 	$(top_srcdir)/m4/ax_append_flag.m4 \
 	$(top_srcdir)/m4/ax_cflags_warn_all.m4 \
 	$(top_srcdir)/m4/ax_type_socklen_t.m4 \
@@ -156,7 +157,7 @@ am__aclocal_m4_deps = $(top_srcdir)/m4/libtool.m4 \
 	$(top_srcdir)/m4/pkg.m4 $(top_srcdir)/m4/ax_pkg_swig.m4 \
 	$(top_srcdir)/m4/ax_python_devel.m4 \
 	$(top_srcdir)/m4/ax_swig_multi_module_support.m4 \
-	$(top_srcdir)/m4/ax_cxx_compile_stdcxx_11.m4 \
+	$(top_srcdir)/m4/ax_cxx_compile_stdcxx_17.m4 \
 	$(top_srcdir)/m4/ax_gcc_func_attribute.m4 \
 	$(top_srcdir)/m4/ax_gcc_var_attribute.m4 \
 	$(top_srcdir)/tests/configure.m4 $(top_srcdir)/configure.ac
@@ -1062,7 +1063,7 @@ ENABLE_THREADS = @ENABLE_THREADS@
 EXEEXT = @EXEEXT@
 FGREP = @FGREP@
 GREP = @GREP@
-HAVE_CXX11 = @HAVE_CXX11@
+HAVE_CXX17 = @HAVE_CXX17@
 INSTALL = @INSTALL@
 INSTALL_DATA = @INSTALL_DATA@
 INSTALL_PROGRAM = @INSTALL_PROGRAM@

acinclude.m4

@@ -17,6 +17,6 @@ m4_include([m4/pkg.m4])
 m4_include([m4/ax_pkg_swig.m4])
 m4_include([m4/ax_python_devel.m4])
 m4_include([m4/ax_swig_multi_module_support.m4])
-m4_include([m4/ax_cxx_compile_stdcxx_11.m4])
+m4_include([m4/ax_cxx_compile_stdcxx_17.m4])
 m4_include([m4/ax_gcc_func_attribute.m4])
 m4_include([m4/ax_gcc_var_attribute.m4])

aclocal.m4

@@ -1467,6 +1467,7 @@ AC_SUBST([am__tar])
 AC_SUBST([am__untar])
 ]) # _AM_PROG_TAR
 
+m4_include([m4/ax_cxx_compile_stdcxx.m4])
 m4_include([m4/libtool.m4])
 m4_include([m4/ltoptions.m4])
 m4_include([m4/ltsugar.m4])