build: Fix check-for-missing for srcdir != builddir

tools/check-for-missing.py

@@ -12,12 +12,13 @@ srcdir=sys.argv[1]
 distdir=sys.argv[2]
 excludes=sys.argv[3:]
 
+cwd=os.getcwd()
 os.chdir(srcdir)
 
 status=0
 for f in subprocess.Popen(["git", "ls-files"], stdout=subprocess.PIPE).stdout:
     f = f.decode('utf-8').strip()
-    if (not os.path.exists(os.path.join(distdir, f)) and
+    if (not os.path.exists(os.path.join(cwd, distdir, f)) and
         not any((fnmatch.fnmatch(f, p) for p in excludes))):
         print("File missing from distribution:", f)
         status=1