build: support LTO in depends

No Qt for now.

depends/README.md

@@ -117,6 +117,7 @@ The following can be set when running make: `make FOO=bar`
 - `LOG`: Use file-based logging for individual packages. During a package build its log file
   resides in the `depends` directory, and the log file is printed out automatically in case
   of build error. After successful build log files are moved along with package archives
+- `LTO`: Use LTO when building packages.
 
 If some packages are not built, for example `make NO_WALLET=1`, the appropriate
 options will be passed to bitcoin's configure. In this case, `--disable-wallet`.