Rollup merge of #39775 - mina86:master, r=steveklabnik
Rollup merge of #39775 - mina86:master, r=steveklabnik

book: don’t use GNU extensions in the example unnecessarily

The use of a GNU C extension for bloc expressions is immaterial to the
actual problem with C macros that the section tries to show so don’t
use it and instead use a plain C way of writing the macro which has
added benefit of being better C code (since the macro now behaves like
a function, syntax-wise).

aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust