upgraded to C++17 compatibility

ax_init_standard_project.m4

@@ -170,9 +170,11 @@ AC_DEFUN([AX_INIT_STANDARD_PROJECT], [
   AX_SUBST(NUMBERS)
   AX_SUBST(HOME)
   if test -f README.md; then
+    README_FILE=README.md
     README=$(tail -n +3 README.md)
     DESCRIPTION=$(head -1 README.md | sed 's,^#\+ *,,;s, *#\+$,,')
   else
+    README_FILE=README
     README=$(tail -n +3 README)
     DESCRIPTION=$(head -1 README)
   fi
@@ -182,6 +184,7 @@ AC_DEFUN([AX_INIT_STANDARD_PROJECT], [
   else
     README_HTML="${README}"
   fi
+  AX_SUBST(README_FILE)
   AX_SUBST(README)
   _AM_SUBST_NOTMAKE([README])
   AX_SUBST(README_ESCAPED)
@@ -329,7 +332,7 @@ AC_DEFUN([AX_INIT_STANDARD_PROJECT], [
   AX_ADD_MAKEFILE_TARGET_DEP([maintainer-clean-am], [maintainer-clean-standard-project-targets], [makefile.in])
   test -f makefile.in && cat >> makefile.in <<EOF
 #### Begin: Appended by $0
-EXTRA_DIST += bootstrap.sh ax_init_standard_project.m4 ax_cxx_compile_stdcxx_11.m4 \
+EXTRA_DIST += bootstrap.sh ax_init_standard_project.m4 ax_cxx_compile_stdcxx.m4 \
               ax_check_qt.m4 resolve-debbuilddeps.sh resolve-rpmbuilddeps.sh \
               build-resource-file.sh mac-create-app-bundle.sh
 
@@ -347,9 +350,9 @@ EOF
 
 # use this in configure.ac to support C++
 AC_DEFUN([AX_USE_CXX], [
-  m4_include(ax_cxx_compile_stdcxx_11.m4)
+  m4_include(ax_cxx_compile_stdcxx.m4)
   AC_LANG(C++)
-  AX_CXX_COMPILE_STDCXX_14(noext, optional)
+  AX_CXX_COMPILE_STDCXX([17], [noext], [optional])
   AC_PROG_CXX
   AC_PROG_CPP