allow some c99

pep-0007.txt

@@ -31,9 +31,22 @@ particular rule:
 C dialect
 =========
 
-* Use ANSI/ISO standard C (the 1989 version of the standard).  This
+* Python versions before 3.6 use ANSI/ISO standard C (the 1989 version
-  means (amongst many other things) that all declarations must be at
+  of the standard).  This means (amongst many other things) that all
-  the top of a block (not necessarily at the top of function).
+  declarations must be at the top of a block (not necessarily at the
+  top of function).
+
+* Python versions greater than or equal to 3.6 use C89 with several
+  select C99 features:
+
+  - Standard integer types in ``<stdint.h>`` and ``<inttypes.h>``
+  - ``static inline`` functions
+  - designated initializers (especially nice for type declarations)
+  - intermingled declarations
+  - booleans
+
+  Future C99 features may be added to this list in the future
+  depending on compiler support (mostly significantly MSVC).
 
 * Don't use GCC extensions (e.g. don't write multi-line strings
   without trailing backslashes).