Autoconf: Better Unicode Python support in makedep

The `open()` commands in `makedep` for reading Fortran source now
includes an `errors=` argument for catching bytes outside of the file
character set.  Unknown characters are replaced with the "unknown"
character (usually �) rather than raising an error.

This avoids problems with Unicode characters and older Pythons which do not
support them, as well as characters from legacy encodings which can
cause errors in Unicode.

Substitution does not break any behavior, since Unicode is only
permitted inside of comment blocks and strings.

This fixes several errors which were silent in `.testing` but were
observed by some users which using autoconf to build their own
executables.

ac/makedep

@@ -4,9 +4,10 @@ from __future__ import print_function
 
 import argparse
 import glob
+import io
 import os
 import re
-import sys  # used only to get path to current script
+import sys
 
 
 # Pre-compile re searches
@@ -255,7 +256,7 @@ def scan_fortran_file(src_file):
     """Scan the Fortran file "src_file" and return lists of module defined,
     module used, and files included."""
     module_decl, used_modules, cpp_includes, f90_includes, programs = [], [], [], [], []
-    with open(src_file, 'r') as file:
+    with io.open(src_file, 'r', errors='replace') as file:
         lines = file.readlines()
         for line in lines:
             match = re_module.match(line.lower())