Trouble getting libobjc2 2.x to build

[Larry Campbell]

This is on Ubuntu Focal. I have two stumbling blocks:

ANNOUNCE.2.1 says:

> The submodule is available from:
> 
> https://github.com/Tessil/robin-map/archive/757de82.zip
> https://github.com/Tessil/robin-map/archive/757de82.tar.gz
> 
> This will extract as robin-map-757de829927489bee55ab02147484850c687b620.
> You must move the contents of that directory into third_party/robin_map in the
> libobjc2 tree.
> 


However, when I follow the instructions and say "cmake .." from the Build 
directory, I get this warning:

> CMake Warning at CMakeLists.txt:118 (find_package):
>   By not providing "Findtsl-robin-map.cmake" in CMAKE_MODULE_PATH this
>   project has asked CMake to find a package configuration file provided by
>   "tsl-robin-map", but CMake did not find one.
> 
>   Could not find a package configuration file provided by "tsl-robin-map"
>   with any of the following names:
> 
>     tsl-robin-mapConfig.cmake
>     tsl-robin-map-config.cmake
> 
>   Add the installation prefix of "tsl-robin-map" to CMAKE_PREFIX_PATH or set
>   "tsl-robin-map_DIR" to a directory containing one of the above files.  If
>   "tsl-robin-map" provides a separate development package or SDK, be sure it
>   has been installed.

Am I missing a step?

The second problem is that I get this warning:

> CMake Warning at CMakeLists.txt:6 (message):
>   WARNING: It is strongly recommended that you compile with clang


The README.md file says:

> If you have gcc and clang both installed, then cmake currently defaults to
> selecting gcc.  You should override this by adding `-DCMAKE_C_COMPILER=clang
> -DCMAKE_CXX_COMPILER=clang++` to your Objective-C flags.


But where do I put these flags? Somewhere in CMakeLists.txt? In one of the 
numerous *.cmake files?

- lc