By 'solution' I mean a solution to the issue mentioned in the title, e.g. building OpenLDAP for Windows. With the instructions provided in that URL I was able to successfully (that is, without build or any other type of errors) build OpenLDAP library in Windows. Even better, by doing according to those instructions I was able to do it by using tools that are kind of 'native' to Windows (like Visual Studio command prompt), not tools that try to emulate some other operating system (like Cygwin or Msys). I needed this so that I can use it from my C++ application developed in Visual Studio through the OpenLDAP api, e.g. by calling functions declared in ldap.h. I don't quite understand how any backends would be related to this...? If I recall correctly, two dependent libraries were required during the linking phase (I think they were OpenSSL and a regex lib), but that became solved by providing the linker with a path to pre-built versions of them (the question concerned only building OpenLDAP, not the dependencies).
ps. English is not my native language, so please be kind if there are that sort of inaccuracies in my text.