Revert #737. Closes #773.

mesonbuild/backend/backends.py

@@ -599,6 +599,25 @@ class Backend():
                                   os.path.join(lead_dir,
                                                outdir))
                 cmd.append(i)
+        # This should not be necessary but removing it breaks
+        # building GStreamer on Windows. The underlying issue
+        # is problems with quoting backslashes on Windows
+        # which is the seventh circle of hell. The downside is
+        # that this breaks custom targets whose command lines
+        # have backslashes. If you try to fix this be sure to
+        # check that it does not break GST.
+        #
+        # The bug causes file paths such as c:\foo to get escaped
+        # into c:\\foo.
+        #
+        # Unfortunately we have not been able to come up with an
+        # isolated test case for this so unless you manage to come up
+        # with one, the only way is to test the building with Gst's
+        # setup. Note this in your MR or ping us and we will get it
+        # fixed.
+        #
+        # https://github.com/mesonbuild/meson/pull/737
+        cmd = [i.replace('\\', '/') for i in cmd]
         return (srcs, ofilenames, cmd)
 
     def run_postconf_scripts(self):