Expose definable definitions to make in Makefile.

Expose definable definitions to make in Makefile.

There are some definitions* in Makefile that seem to be exposed to external definition but there is no way to pass them to Makefile.
Can you add some variable like \`USER_CFLAGS' so user could pass additional options to Makefile and not have to manually change command line or edit the file.
And describe example in the Readme.

Or, if it's already possible describe it in Readme.

* like ZSTDCLI_CLEVEL_DEFAULT, ZSTDCLI_NBTHREADS_DEFAULT, ZSTD_NOBENCH, etc.

----

Also a request to enclose DISPLAY_LEVEL_DEFAULT in \`#ifdef ... #endif', so it too could be passed through makefile.
```
+#ifndef DISPLAY_LEVEL_DEFAULT
#define DISPLAY_LEVEL_DEFAULT 2
+#endif
```
Here's a [diff](https://dpaste.com/9AGC8ZEQA.txt) for convenience. Just add a new line at the end.