Uses CXX17 if it cannot find timespec_get or TIME_UTC with CXX11. Fixes

#115

configure

@@ -20,9 +20,37 @@ else
   EXTRA_PKG_LIBS=-latomic
 fi
 
+# Detect whether it requires c++17
+echo "#include <cstdio>
+#include <ctime>
+// Example adapted from https://en.cppreference.com/w/cpp/chrono/c/timespec_get
+
+namespace std {
+
+	int main()
+	{
+		timespec ts;
+		timespec_get(&ts, TIME_UTC);
+		char buf[100];
+		strftime(buf, sizeof buf, "%D %T", gmtime(&ts.tv_sec));
+		printf("Current time: %s.%09ld UTC\n", buf, ts.tv_nsec);
+		return 0;
+	}
+
+}
+" | ${CXX} -x c++ -std=c++11 - -o /dev/null > /dev/null 2>&1
+if [ $? -eq 0 ]; then
+  echo "c++11 is enough."
+  CXX_STD=CXX11
+else
+  echo "c++17 required."
+  CXX_STD=CXX17
+  # See https://github.com/r-lib/later/issues/115
+fi
+
 
 # Write to Makevars
-sed -e "s|@extra_pkg_libs@|$EXTRA_PKG_LIBS|" src/Makevars.in > src/Makevars
+sed -e "s|@extra_pkg_libs@|$EXTRA_PKG_LIBS|;s|@cxx_std@|$CXX_STD|" src/Makevars.in > src/Makevars
 
 # Success
 exit 0

src/Makevars.in

@@ -1,5 +1,4 @@
-# Use C++11 if available
-CXX_STD=CXX11
+CXX_STD=@cxx_std@
 
 PKG_CPPFLAGS = -pthread -DSTRICT_R_HEADERS
 PKG_LIBS = -pthread @extra_pkg_libs@