How to compile & install libc++ on Linux

Problem:

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)

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