add clarity for custom path installation

install.sysconfdir is another value, in addition to install.prefix,
that could be set for custom path installation.
Signed-off-by: Naveen R. Iyer <iyernaveenr@gmail.com>

Author: iyernaveenr

INSTALL.md

@@ -79,9 +79,23 @@ See [the rustc-dev-guide for more info][sysllvm].
  ./configure
 ```
 
- If you plan to use `x.py install` to create an installation, it is
- recommended that you set the `prefix` value in the `[install]` section to a
- directory: `./configure --set install.prefix=<path>`
+ If you plan to use `x.py install` to create an installation, you can either
+ set `DESTDIR` environment variable to your custom directory path:
+
+```bash
+export DESTDIR=<path>
+```
+
+ or set `prefix` and `sysconfdir` in the `[install]` section to your custom
+ directory path:
+
+```sh
+ ./configure --set install.prefix=<path> --set install.sysconfdir=<path>
+```
+
+ When the `DESTDIR` environment variable is present, the `prefix` and
+`sysconfdir` values are combined with the path from the `DESTDIR`
+ environment variable.
 
 3. Build and install: