message: CI: Explain.
diff: @@ -9,6 +9,7 @@ x86_64-centos6-gcc4:
  stage: build
  script:
  - export DRAKE_MTIME=0 # rsync and mtime comparisons don't play along very well
  + - export DRAKE_EXPLAIN=1
  - git submodule sync
  - git submodule update --init --recursive
  - git fetch origin --tags
  @@ -39,6 +40,7 @@ x86_64-ubuntu-gcc4:
  stage: build
  script:
  - export DRAKE_MTIME=0 # rsync and mtime comparisons don't play along very well
  + - export DRAKE_EXPLAIN=1
  - git submodule sync --recursive
  - git submodule update --init --recursive
  - git fetch origin --tags
  @@ -70,6 +72,7 @@ x86_64-jessie-gcc4:
  stage: build
  script:
  - export DRAKE_MTIME=0 # rsync and mtime comparisons don't play along very well
  + - export DRAKE_EXPLAIN=1
  - git submodule sync --recursive
  - git submodule update --init --recursive
  - git fetch origin --tags
  @@ -100,6 +103,7 @@ x86_64-alpine-gcc:
  stage: build
  script:
  - export DRAKE_MTIME=0 # rsync and mtime comparisons don't play along very well
  + - export DRAKE_EXPLAIN=1
  - git submodule sync --recursive
  - git submodule update --init --recursive
  - git fetch origin --tags
  @@ -131,6 +135,7 @@ i386-jessie-gcc4:
  stage: build
  script:
  - export DRAKE_MTIME=0 # rsync and mtime comparisons don't play along very well
  + - export DRAKE_EXPLAIN=1
  - git submodule sync --recursive
  - git submodule update --init --recursive
  - git fetch origin --tags
  @@ -161,6 +166,7 @@ x86_64-windows-mingw4:
  stage: build
  script:
  - export DRAKE_MTIME=0 # rsync and mtime comparisons don't play along very well
  + - export DRAKE_EXPLAIN=1
  - git submodule sync --recursive
  - git submodule update --init --recursive
  - git fetch origin --tags
  @@ -191,6 +197,7 @@ x86_64-windows-mingw4:
  # stage: build
  # script:
  # - export DRAKE_MTIME=0 # rsync and mtime comparisons don't play along very well
  +# - export DRAKE_EXPLAIN=1
  # - git submodule sync --recursive
  # - git submodule update --init --recursive
  # - git fetch origin --tags
  @@ -207,7 +214,6 @@ x86_64-windows-mingw4:
  x86_64-osx-clang:
  stage: build
  script:
  - - export DRAKE_MTIME=0 # rsync and mtime comparisons don't play along very well
  - git submodule sync --recursive
  - git submodule update --init --recursive
  - git fetch origin --tags
  