Bump C++ standard to C++17 (autotools)

This is necessary if we want to use googletest 1.17.0

Author: d-torrance

M2/configure.ac

@@ -262,7 +262,7 @@ AC_SUBST(M2_CFLAGS)
 AC_SUBST(M2_CXXFLAGS)
 
 dnl Fix standards, apply everywhere
-CXXFLAGS="-std=gnu++14 $CXXFLAGS"
+CXXFLAGS="-std=gnu++17 $CXXFLAGS"
 CFLAGS="-std=gnu11 $CFLAGS"
 
 AC_PROG_CC() 			# set CFLAGS before this