Add 'proc_macro' to configuation options

Author: Havvy

src/conditional-compilation.md

@@ -84,6 +84,8 @@ The following configurations must be defined by the implementation:
   This can be used to enable extra debugging code in development but not in
   production.  For example, it controls the behavior of the standard library's
   `debug_assert!` macro.
+* `proc_macro` - Set when the crate being compiled is being compiled with the
+  `proc_macro` [crate type].
 
 You can also set another [attribute] based on a `cfg` variable with `cfg_attr`:
 
@@ -96,4 +98,5 @@ This is the same as `#[b]` if `a` is set by `cfg`, and nothing otherwise.
 Lastly, configuration options can be used in expressions by invoking the `cfg!`
 macro: `cfg!(a)` evaluates to `true` if `a` is set, and `false` otherwise.
 
-[attribute]: attributes.html
+[attribute]: attributes.html
+[crate type]: linkage.html