provide a clearer error message in case Popen'ed executable is not executable

subprocess.Popen() will give a proper error message if the first
argument is an absolute path, but if it's a bare command name, it will
complain with a "Permission Denied" error.

This change provides a clear error every time.

If speed is proven to have an impact on this code, we could add
the "Logs.verbose > 0" precondition or add memoization.

waflib/Context.py

@@ -327,6 +327,9 @@ class Context(ctx):
 		if 'stderr' not in kw:
 			kw['stderr'] = subprocess.PIPE
 
+		if not kw['shell'] and Utils.check_exe(cmd[0]) is None:
+			raise Errors.WafError("Program %s not found!" % cmd[0])
+
 		try:
 			if kw['stdout'] or kw['stderr']:
 				p = subprocess.Popen(cmd, **kw)
@@ -388,6 +391,9 @@ class Context(ctx):
 		else:
 			to_ret = STDOUT
 
+		if not kw['shell'] and Utils.check_exe(cmd[0]) is None:
+			raise Errors.WafError("Program %s not found!" % cmd[0])
+
 		kw['stdout'] = kw['stderr'] = subprocess.PIPE
 		if quiet is None:
 			self.to_log(cmd)

waflib/Utils.py

@@ -440,8 +440,8 @@ def check_dir(path):
 	"""
 	Ensure that a directory exists (similar to ``mkdir -p``).
 
-	:type  dir: string
-	:param dir: Path to directory
+	:type  path: string
+	:param path: Path to directory
 	"""
 	if not os.path.isdir(path):
 		try:
@@ -450,6 +450,28 @@ def check_dir(path):
 			if not os.path.isdir(path):
 				raise Errors.WafError('Cannot create the folder %r' % path, ex=e)
 
+def check_exe(name):
+	"""
+	Ensure that a program exists
+	:type name: string
+	:param name: name or path to program
+	:return: path of the program or None
+	"""
+	def is_exe(fpath):
+		return os.path.isfile(fpath) and os.access(fpath, os.X_OK)
+
+	fpath, fname = os.path.split(name)
+	if fpath and is_exe(name):
+		return fpath
+	else:
+		for path in os.environ["PATH"].split(os.pathsep):
+			path = path.strip('"')
+			exe_file = os.path.join(path, name)
+			if is_exe(exe_file):
+				return exe_file
+
+	return None
+
 def def_attrs(cls, **kw):
 	"""
 	Set default attributes on a class instance