It is not upstream business to decide what build style we can use specially when they don't cover all the accpetable values from CMake itself.

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 2bfb6d5..286d3a9 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -13,20 +13,6 @@ endif()
 set(Eigen_SOURCE_DIR ${Eigen3_SOURCE_DIR})
 set(Eigen_BINARY_DIR ${Eigen3_BINARY_DIR})
 
-# guard against bad build-type strings
-
-if (NOT CMAKE_BUILD_TYPE)
-  set(CMAKE_BUILD_TYPE "Release")
-endif()
-
-string(TOLOWER "${CMAKE_BUILD_TYPE}" cmake_build_type_tolower)
-if(    NOT cmake_build_type_tolower STREQUAL "debug"
-   AND NOT cmake_build_type_tolower STREQUAL "release"
-   AND NOT cmake_build_type_tolower STREQUAL "relwithdebinfo")
-  message(FATAL_ERROR "Unknown build type \"${CMAKE_BUILD_TYPE}\". Allowed values are Debug, Release, RelWithDebInfo (case-insensitive).")
-endif()
-
-
 #############################################################################
 # retrieve version infomation                                               #
 #############################################################################