Quote CMAKE_BUILD_TYPE in CMakeLists.txt to allow it being unset.

[janbraun]

Right now, cmake will fail when called without defining CMAKE_BUILD_TYPE with

CMake Error at CMakeLists.txt:15 (if):
  if given arguments:

    "STREQUAL" "Debug"

  Unknown arguments specified

Quoting the variable will make cmake pass. This is super-trivial and maybe not what you want.
Then I would suggest to set a default build type instead, or ignoring this pull request. :-)

[Yours3lf]

Thank you for noticing this! Kind of on the fence about this, the error will probably make people actually read build.md :)