cmake BUGFIX do not error out on custom CMAKE_BUILD_TYPE

* Discovered while trying to package version 2.0.0 on Gentoo Linux,
  where we use `CMAKE_BUILD_TYPE=Gentoo`, so don't error out on
  `CMAKE_BUILD_TYPE` different than the ones specified in `CMakeLists.txt`
diff --git a/CMakeLists.txt b/CMakeLists.txt
index fbe6c33..c16383f 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -35,9 +35,7 @@
   set(CMAKE_BUILD_TYPE "ABICheck" CACHE STRING "Build Type" FORCE)
 elseif ("${BUILD_TYPE_UPPER}" STREQUAL "DOCONLY")
   set(CMAKE_BUILD_TYPE "DocOnly" CACHE STRING "Build Type" FORCE)
-else ()
-  message(FATAL_ERROR "Unknown CMAKE_BUILD_TYPE \"${CMAKE_BUILD_TYPE}\".")
-endif ()
+endif()
 
 # check the supported platform
 if(NOT UNIX)