config.toml: Add stubs for recognized-but-unused install paths

... specifically `datadir`, `infodir`, and `localstatedir`.  These were
already accepted by `configure.py`, but it didn't have any place to put
the values.

Author: cuviper

config.toml.example

@@ -203,6 +203,16 @@
 # Where to install man pages in `prefix` above
 #mandir = "share/man"
 
+# Where to install data in `prefix` above (currently unused)
+#datadir = "share"
+
+# Where to install additional info in `prefix` above (currently unused)
+#infodir = "share/info"
+
+# Where to install local state (currently unused)
+# If this is a relative path, it will get installed in `prefix` above
+#localstatedir = "/var/lib"
+
 # =============================================================================
 # Options for compiling Rust code itself
 # =============================================================================

src/bootstrap/config.rs

@@ -207,6 +207,11 @@ struct Install {
     bindir: Option<String>,
     libdir: Option<String>,
     mandir: Option<String>,
+
+    // standard paths, currently unused
+    datadir: Option<String>,
+    infodir: Option<String>,
+    localstatedir: Option<String>,
 }
 
 /// TOML representation of how the LLVM build is configured.