[compiler-rt] [runtimes] Allow building against an installed LLVM tree (PR #86209)

[Alexander Richardson via llvm-commits]

arichardson wrote:

> Basically the steps in https://libcxx.llvm.org/VendorDocumentation.html#vendordocumentation https://libcxx.llvm.org/TestingLibcxx.html
> 
> ```
> cmake -G "Unix Makefiles" -S runtimes -B build_flat_multimap -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi;libunwind" -DLIBCXX_INCLUDE_TESTS:BOOL=TRUE -DLIBCXX_TEST_PARAMS="enable_modules=clang" -DCMAKE_CXX_COMPILER="/usr/local/bin/clang++" -DPython3_EXECUTABLE="/usr/local/bin/python"
> 
> cd build_flat_multimap
> make cxx cxxabi cxx_experimental -j12
> make cxx-test-depends
> ./bin/llvm-lit -sv  --param std=c++23  ../libcxx/test/std
> ```
> 
> The output of `make -n check-all`
> 
> ```
> make -n check-all
> make: *** No rule to make target `check-all'.  Stop.
> ```

Thanks, I'll take a look and will try to fix this. Unrelated but if you use -G Ninja the build should be a little bit faster.

https://github.com/llvm/llvm-project/pull/86209