You want to compile and install libc++ (sometimes also named libcxx), but CMake complains with this error message
CMake Error at cmake/Modules/MacroEnsureOutOfSourceBuild.cmake:7 (message): libcxx requires an out of source build. Please create a separate</em> build directory and run 'cmake /path/to/libcxx [options]' there. Call Stack (most recent call first): CMakeLists.txt:24 (MACRO_ENSURE_OUT_OF_SOURCE_BUILD) CMake Error at cmake/Modules/MacroEnsureOutOfSourceBuild.cmake:8 (message): In-source builds are not allowed. CMake would overwrite the makefiles distributed with Compiler-RT. Please create a directory and run cmake from there, passing the path to this source directory as the last argument. This process created the file `CMakeCache.txt' and the directory `CMakeFiles'. Please delete them. Call Stack (most recent call first): CMakeLists.txt:24 (MACRO_ENSURE_OUT_OF_SOURCE_BUILD)
Once you know what “out-of-source-build” means, it’s actually pretty easy: You must not execute cmake from the libcxx directory itself, you need to execute it from any parent directory – the build will take place there.
Just run this script in your favourite shell:
mkdir libc++ svn co http://llvm.org/svn/llvm-project/libcxx/trunk libcxx cmake libcxx make
This creates the directory libc++ inside your current directory and downloads and compiles the latest libc++ SVN trunk inside it. Run
sudo make install inside the libc++ directory (not inside libcxx!) to install it globally.