If you try to disable in-source builds, cmake creates CMakeCache.txt and a CMakeFiles folder after the fatal error stops the script. These files then prevent the user from doing an out-of-source build. It’s worthwhile to explain this ridiculous situation to the user, since the script itself can’t solve the problem:

# Do this before project() to skip the compiler config process
if ("${CMAKE_BINARY_DIR}" STREQUAL "${CMAKE_SOURCE_DIR}") 
  message(FATAL_ERROR "In-source builds are disabled.
    Please create a subfolder and use `cmake ..` inside it.
    NOTE: cmake will now create CMakeCache.txt and CMakeFiles/*.
          You must delete them, or cmake will refuse to work.")
endif() # yes, end-markers and even else() need empty parens

At some point (not sure which version) cmake reportedly added a secret setting to disable in-source builds.

set(CMAKE_DISABLE_IN_SOURCE_BUILD ON)

But this has the same problem (CMakeCache.txt and CMakeFiles are created) and gives users a cryptic error message.

Interestingly my version of cmake (3.11) doesn’t document the -H option for setting the source folder (or the -B option for setting the binaries folder):

cmake -H. -Bbuild

In fact, cmake --help claims that -H is a synonym for --help. Nevertheless the command seems to work :)