updated: https://github.com/ceph/ceph/commit/6e4481316884f08daad624c1d997378daedf410e

commit a7e3ece459111d157a20d05de3a92cf4dab6bde6
Author: Kefu Chai <kchai@redhat.com>
Date:   Thu Jul 1 15:24:50 2021 +0800

    cmake: set CMAKE_BUILD_TYPE only if .git exists
    
    distros intend to fine tune the CFLAGS and CXXFLAGS by themselves, see
    
    - https://git.alpinelinux.org/abuild/tree/abuild.conf
    - https://wiki.archlinux.org/title/CMake_package_guidelines#CMake_undesired_behaviors
    - https://github.com/Debian/debhelper/blob/5d1bb29841043d8e47ebbdd043e6cd086cad508e/lib/Debian/Debhelper/Buildsystem/cmake.pm#L16
    
    so instead of setting a CMAKE_BUILD_TYPE when building from a
    dist tarball, let's just leave it empty.
    
    Signed-off-by: Kefu Chai <kchai@redhat.com>

diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 760a2ceb0c..6f35e87f90 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -156,12 +156,8 @@ elseif(CMAKE_CXX_COMPILER_ID STREQUAL Clang)
   endif()
 endif(CMAKE_CXX_COMPILER_ID STREQUAL GNU)
 
-if(NOT CMAKE_BUILD_TYPE)
-  if(EXISTS "${CMAKE_SOURCE_DIR}/.git")
-    set(default_build_type "Debug")
-  else()
-    set(default_build_type "RelWithDebInfo")
-  endif()
+if(NOT DEFINED CMAKE_BUILD_TYPE AND EXISTS "${CMAKE_SOURCE_DIR}/.git")
+  set(default_build_type "Debug")
   set(CMAKE_BUILD_TYPE "${default_build_type}" CACHE
       STRING "Default BUILD_TYPE is Debug, other options are: RelWithDebInfo, Release, and MinSizeRel." FORCE)
 endif()