Configure gerrit hook at CMake time

If the source directory is a clone, at CMake time the commit-msg hook gets
downloaded and installed in the right location.

Change-Id: I5fee17d050ca22d8b92a49fdcc2a1cd6659f209b

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index c6cce47..afc8fe5 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -36,6 +36,32 @@
 
 PROJECT(CERES C CXX)
 
+# Set up the git hook to make Gerrit Change-Id: lines in commit messages.
+SET (LOCAL_GIT_DIRECTORY)
+IF (EXISTS ${CMAKE_SOURCE_DIR}/.git)
+  # .git directory can be found on Unix based system, or on Windows with
+  # Git Bash (shipped with msysgit)
+  SET (LOCAL_GIT_DIRECTORY ${CMAKE_SOURCE_DIR}/.git)
+ELSE (EXISTS ${CMAKE_SOURCE_DIR}/.git)
+  # TODO(keir) Add proper windows support
+ENDIF (EXISTS ${CMAKE_SOURCE_DIR}/.git)
+
+IF (EXISTS ${LOCAL_GIT_DIRECTORY})
+  IF (NOT EXISTS ${LOCAL_GIT_DIRECTORY}/hooks/commit-msg)
+    # Download the hook only if it is not already present
+    FILE(DOWNLOAD https://ceres-solver-review.googlesource.com/tools/hooks/commit-msg
+         ${CMAKE_BINARY_DIR}/commit-msg)
+
+    # Make the downloaded file executable, since it is not by default.
+    FILE(COPY ${CMAKE_BINARY_DIR}/commit-msg
+         DESTINATION ${LOCAL_GIT_DIRECTORY}/hooks/
+         FILE_PERMISSIONS
+           OWNER_READ OWNER_WRITE OWNER_EXECUTE
+           GROUP_READ GROUP_WRITE GROUP_EXECUTE
+           WORLD_READ WORLD_EXECUTE)
+  ENDIF (NOT EXISTS ${LOCAL_GIT_DIRECTORY}/hooks/commit-msg)
+ENDIF (EXISTS ${LOCAL_GIT_DIRECTORY})
+
 SET(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
 SET(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
 SET(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)