open_spiel: add_subdirectory given source "abseil-cpp" which is not existing

Running (patched – see #2) ./open_spiel/scripts/build_and_run_tests.sh per install instructions results in the following:

++ CXX=g++
++ NPROC=nproc
++ [[ darwin18 == \d\a\r\w\i\n* ]]
++ NPROC='sysctl -n hw.physicalcpu'
++ CXX=/usr/local/bin/g++-7
+++ sysctl -n hw.physicalcpu
++ MAKE_NUM_PROCS=4
++ let 'TEST_NUM_PROCS=4*4'
+++ python3 -c 'import sys; print(sys.version.split(" ")[0])'
++ PYVERSION=3.6.5
+++ python3 -c 'import sys; print(sys.version_info.major)'
++ PY_VERSION_MAJOR=3
++ BUILD_DIR=build_python_3
++ mkdir -p build_python_3
++ cd build_python_3
/Users/howes/src/open_spiel/build_python_3
++ echo 'Building and testing in /Users/howes/src/open_spiel/build_python_3 using '\''python'\'' (version 3.6.5).'
Building and testing in /Users/howes/src/open_spiel/build_python_3 using 'python' (version 3.6.5).
++ cmake -DPython_TARGET_VERSION=3.6.5 -DCMAKE_CXX_COMPILER=/usr/local/bin/g++-7 ../open_spiel
-- The C compiler identification is AppleClang 10.0.1.10010046
-- The CXX compiler identification is GNU 7.4.0
-- Check for working C compiler: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/cc
-- Check for working C compiler: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Checking whether CXX compiler has -isysroot
-- Checking whether CXX compiler has -isysroot - yes
-- Checking whether CXX compiler supports OSX deployment target flag
-- Checking whether CXX compiler supports OSX deployment target flag - yes
-- Check for working CXX compiler: /usr/local/bin/g++-7
-- Check for working CXX compiler: /usr/local/bin/g++-7 -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
CMake Error at CMakeLists.txt:45 (add_subdirectory):
  add_subdirectory given source "abseil-cpp" which is not an existing
  directory.


-- Found Python3: /usr/local/opt/python/Frameworks/Python.framework/Versions/3.6/lib/libpython3.6m.dylib (found version "3.6.5") found components:  Development 
-- Configuring incomplete, errors occurred!
See also "/Users/howes/src/open_spiel/build_python_3/CMakeFiles/CMakeOutput.log".