add postfix for different types of builds

[mitkof6]

Currently when docopt is installed for different build types the last build overrides the libraries. This PR adds a default postfix based on the build type.

issue

[jaredgrubb]

I'm not experienced with cmake (one day I'll learn it), so I'd like to have someone volunteer to look at this?

[mitkof6]

Hi @jaredgrubb. The set command assigns a value to the variable. The postfix variables are used when the library is build so you have something like this docopt_[d|r|rd|md].[lib|dll|so]. The mark_as advanced command hides the cache variable under the advanced option, which can be easily accessed and changed during configuration. Finally, when you install the library the different builds are distinguished by the postfix. Since you have correctly configured the install process a separate cmake target is constructed for each build type so that the library can find the corresponding files and link them correctly when used by a third party.