agones: sdk/cpp cmake build fails on Linux

This issue came up during #713, but I’m moving the relevant details into an issue so it can be tracked separately.

My system:

• Debian Linux, rolling release #1 SMP Debian 4.19.28-2rodete1 (2019-03-18 > 2018)
• cmake version 3.12.1
• grpc is not installed locally on my machine

Here are the commands I ran in this repo and the output:

$ cd sdks/cpp/
$ cd build_scripts/
$ bash build.sh 
-- git Version: v0.0.0-dirty
-- Version: 0.0.0
-- Performing Test HAVE_STD_REGEX
-- Performing Test HAVE_STD_REGEX -- success
-- Performing Test HAVE_GNU_POSIX_REGEX
-- Performing Test HAVE_GNU_POSIX_REGEX -- failed to compile
-- Performing Test HAVE_POSIX_REGEX
-- Performing Test HAVE_POSIX_REGEX -- success
-- Performing Test HAVE_STEADY_CLOCK
-- Performing Test HAVE_STEADY_CLOCK -- success
CMake Error at CMakeLists.txt:177 (install):
  install TARGETS given target "address_sorting" which does not exist in this
  directory.


CMake Error at CMakeLists.txt:195 (install):
  install TARGETS given target "address_sorting" which does not exist in this
  directory.


-- Configuring incomplete, errors occurred!
See also "/home/jgm/github/devjgm/agones/sdks/cpp/.build/CMakeFiles/CMakeOutput.log".
See also "/home/jgm/github/devjgm/agones/sdks/cpp/.build/CMakeFiles/CMakeError.log".
make: *** No rule to make target 'install'.  Stop.