This option does not suppress the preprocessor's debug output, such as -dM. To avoid mixing such
debug output with the dependency rules you should explicitly specify the dependency output file with
-MF, or use an environment variable like DEPENDENCIES_OUTPUT. Debug output will still be sent to the
regular output stream as normal.
When used with the driver options -MD or -MMD, -MF overrides the default dependency output file.
-MD-MD is equivalent to -M-MFfile, except that -E is not implied. The driver determines file based on
whether an -o option is given. If it is, the driver uses its argument but with a suffix of .d,
otherwise it takes the name of the input file, removes any directory components and suffix, and
applies a .d suffix.
In other words, this environment variable is equivalent to combining the options -MM and -MF, with an
optional -MT switch too.