Only rerun bindgen when its dependencies change

This took a bit of wrangling to get the depfiles working, but I
eventually figured it out. ninja -d explain is very useful.

Fixed: 597
Change-Id: I909a4c9418e9dc954e3d328da8f3a825e62544e4
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/59005
Reviewed-by: Bob Beck <bbe@google.com>
Auto-Submit: David Benjamin <davidben@google.com>
Commit-Queue: Bob Beck <bbe@google.com>

cmake/go.cmake

diff --git a/cmake/go.cmake b/cmake/go.cmake
index 61a01f5..966ad32 100644
--- a/cmake/go.cmake
+++ b/cmake/go.cmake
@@ -34,10 +34,7 @@
   else()
     # Ninja expects the target in the depfile to match the output. This is a
     # relative path from the build directory.
-    string(LENGTH "${CMAKE_BINARY_DIR}" root_dir_length)
-    math(EXPR root_dir_length "${root_dir_length} + 1")
-    string(SUBSTRING "${CMAKE_CURRENT_BINARY_DIR}" ${root_dir_length} -1 target)
-    set(target "${target}/${dest}")
+    binary_dir_relative_path(${dest} target)
 
     set(depfile "${CMAKE_CURRENT_BINARY_DIR}/${dest}.d")
     add_custom_command(OUTPUT ${dest}