Generate error data at build time.

This avoids cluttering up the diff and making merge conflicts a pain.  It does,
however, mean we need to generate err_data.c ahead of time in Chromium and
likely other downstream builds. It also adds a build dependency on Go.

Change-Id: I6e0513ed9f50cfb030f7a523ea28519590977104
Reviewed-on: https://boringssl-review.googlesource.com/3790
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/BUILDING b/BUILDING
index 67f8a33..8a65528 100644
--- a/BUILDING
+++ b/BUILDING
@@ -19,7 +19,8 @@
     with Platform SDK 8.1 or later are supported. Recent versions of GCC and
     Clang should work on non-Windows platforms, and maybe on Windows too.
 
-  * Go[5] is required for running tests, but not for building.
+  * Go[5] is required. If not found by CMake, the go executable may be
+    configured explicitly by setting GO_EXECUTABLE.
 
 Using Ninja (note the 'N' is capitalized in the cmake invocation):