build: python 3 compatibility

Ubuntu 16.04 "xenial xerus" does not come with Python 2.x by default.
It is possible to install a python-2.7 package, but this has its own
problem: no `python` or `python2` symlink (see #7717).

This fixes the following scripts to work with python 3:
- `make check` (bctest,py, bitcoin-util-test.py)
- `make translate` (extract_strings_qt.py)
- `make symbols-check` (symbol-check.py)
- `make security-check` (security-check.py)

Explicitly call the python commands using $(PYTHON) instead
of relying on the interpreter line at the top of the scripts.

share/qt/extract_strings_qt.py

@@ -1,8 +1,9 @@
 #!/usr/bin/python
 '''
-Extract _("...") strings for translation and convert to Qt4 stringdefs so that
+Extract _("...") strings for translation and convert to Qt stringdefs so that
 they can be picked up by Qt linguist.
 '''
+from __future__ import division,print_function,unicode_literals
 from subprocess import Popen, PIPE
 import glob
 import operator
@@ -52,10 +53,14 @@ files = sys.argv[1:]
 
 # xgettext -n --keyword=_ $FILES
 XGETTEXT=os.getenv('XGETTEXT', 'xgettext')
+if not XGETTEXT:
+    print('Cannot extract strings: xgettext utility is not installed or not configured.',file=sys.stderr)
+    print('Please install package "gettext" and re-run \'./configure\'.',file=sys.stderr)
+    exit(1)
 child = Popen([XGETTEXT,'--output=-','-n','--keyword=_'] + files, stdout=PIPE)
 (out, err) = child.communicate()
 
-messages = parse_po(out) 
+messages = parse_po(out.decode('utf-8'))
 
 f = open(OUT_CPP, 'w')
 f.write("""