INSTALL_BUILD_DEPS option seems not working

**What version of OR-Tools and what language are you using?**
Version: master
Language: C++

**What operating system (Linux, Windows, ...) and version?**
Ubuntu 18.04.5 LTS (Bionic Beaver)

**What did you do?**
I used or-tools inside my project. I set BUILD_DEPS=ON but I did not want to install the built files.
So I set INSTALL_BUILD_DEPS=OFF, but it seems it does nothing and the installation is still happening.
(By searching INSTALL_BUILD_DEPS inside the repository, you will see nothing but the definition itself.)

**What did you expect to see**
By setting `INSTALL_BUILD_DEPS=OFF`, I expect the cmake not to install the programs.

**What did you see instead?**

Error like
```
Install the project...
-- Install configuration: "Release"
-- Installing: /usr/local/lib/libz.a
CMake Error at _deps/zlib-build/cmake_install.cmake:46 (file):
file INSTALL cannot copy file
"/path/to/my/build/lib/libz.a" to
"/usr/local/lib/libz.a": Success.
Call Stack (most recent call first):
_deps/or-tools-build/dependencies/cmake_install.cmake:47 (include)
_deps/or-tools-build/cmake_install.cmake:47 (include)
cmake_install.cmake:57 (include)
```