Misc fixups to satisfy Android NDK unified headers

Summary:
Apply fixes that work with new unified headers and remain compatible with old per-API+arch headers.

I do this to unclutter the diff that enables unified headers, so that it only contains things that are incompatible with old per-API+arch headers.

Reviewed By: yfeldblum

Differential Revision: D10225372

fbshipit-source-id: b92cb3ef45cf23f2233c39520236c4531dba715b

folly/String.cpp

@@ -485,7 +485,14 @@ fbstring errnoStr(int err) {
     result.assign(buf);
   }
 #elif defined(FOLLY_HAVE_XSI_STRERROR_R) || defined(__APPLE__) || \
-    defined(__ANDROID__)
+    (defined(__ANDROID__) &&                                      \
+     !(defined(_STRING_H) && defined(__USE_GNU) && __ANDROID_API__ >= 23))
+  // Above: _STRING_H (no trailing underscore) guards unified NDK headers
+  // vs. _STRING_H_ which guards old per-API+arch NDK headers.
+  // Old non-unified headers always declare strerror_r as XSI compatible.
+  // New unified headers declare it as XSI compatible for API<23,
+  // but as GNU-compatible (with return type char*) for API>=23.
+
   // Using XSI-compatible strerror_r
   int r = strerror_r(err, buf, sizeof(buf));