[CMake] Building gtest with ExternalProject at configure time

[Craig Scott]

Hi all. I was surprised recently to find little in the way of a simple
method to build gtest as part of a CMake project without having to either
(a) add the gtest source to my own project or (b) use ExternalProject and
then have to manually create import libraries for tests to link against.
Since neither option was particularly appealing, I found a way to combine
the best of both approaches without the downsides subsequently wrote a blog
article showing how to do it. The general gist of it is to use
ExternalProject to download at configure time and then add the gtest source
to your project using add_subdirectory(). This means gtest builds with the
same compiler and settings as the rest of your project, but it does not
require adding the gtest source to your project tree. The gtest and
gtest_main CMake targets are automatically available in your project
without having to define any import libraries manually.

The method I used was relatively straightforward, I create a sub-project in
the main project's build area and build it as part of the main CMake step.
All the sub-project does is use ExternalProject to download the gtest
source to a defined location, so the main project then has the source
already in place and can therefore use add_subdirectory() to bring gtest
into your main project. It turns out that it was also easy to make the
approach generic so it could be applied to any external CMake-based
project. The blog article with all the details can be found here:

http://crascit.com/2015/07/25/cmake-gtest/

What struck me after investigating and implementing this was how useful it
might be if ExternalProject had an option which made it run (at least some
steps) at configure time (i.e. when CMake is run) rather than at build
time. My approach seems to suggest it may not be all that hard, but the
ExternalProject implementation is more involved than I wanted to pick
apart. If anyone wants to explore it, my blog article probably shows you
enough of a clue to how it might be possible to do it.

-- 
Craig Scott
Melbourne, Australia
http://crascit.com
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