- Remove hardcoded optimizations.
- Partially revert upstream commit b76d2fb1e5bb839249b25b7094db94219695f515,
  it breaks the build on OpenBSD.

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -283,15 +283,10 @@ else()
   endif()
   add_debug_compile_flag("-O0")
   add_debug_compile_flag("-g3")
-  add_nondebug_compile_flag("-O2")
   if(BYN_ENABLE_ASSERTIONS)
     # On non-Debug builds cmake automatically defines NDEBUG, so we
     # explicitly undefine it:
     add_nondebug_compile_flag("-UNDEBUG")
-  endif()
-  if(NOT APPLE AND NOT "${CMAKE_CXX_FLAGS}" MATCHES "-fsanitize")
-    # This flag only applies to shared libraries so don't use add_link_flag
-    set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -Wl,--no-undefined")
   endif()
 endif()