Improving compilation instructions in COMPILING.md on mac

[HuStmpHrrr]

I followed the instructions in COMPILING.md using cmake and was not able to compile cmbc on mac.

It appeared that the necessary cmake command should also specify a C++ compiler (clang), otherwise C++'s standard library won't be found. The following is the command line I used:

cmake -S. -Bbuild -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DWITH_JBMC=OFF

[tautschnig]

Oh, indeed we are only talking about CMAKE_C_COMPILER. Can you please confirm that it was sufficient to use clang++, or did you have to use /usr/bin/clang++?

[HuStmpHrrr]

I can confirm that it runs without having to specify the whole path. the exact command above works for me.