Use C++17 compile flag when possible (#41)

setup.py

@@ -52,15 +52,15 @@ def has_flag(compiler, flagname):
 
 
 def cpp_flag(compiler):
-    """Return the -std=c++[11/14] compiler flag.
+    """Return the -std=c++[11/14/17] compiler flag.
 
-    The c++14 is prefered over c++11 (when it is available).
+    The newer version is prefered over c++11 (when it is available).
     """
-    if has_flag(compiler, '-std=c++14'):
-        return '-std=c++14'
-    elif has_flag(compiler, '-std=c++11'):
-        return '-std=c++11'
-    else:
+    flags = ['-std=c++17', '-std=c++14', '-std=c++11']
+
+    for flag in flags:
+        if has_flag(compiler, flag): return flag
+
     raise RuntimeError('Unsupported compiler -- at least C++11 support '
                        'is needed!')