add option to be able to disable example executables.

[jothepro]

Reasons why to disable the example-executable configuration:

• The example should not be built by hunter, because it is not part of the library and I don't want to waste compile-time on it.
• I had trouble building the example while cross-compiling to iOS

For "backwards compatibility" reasons I kept the option enabled by default.

[NeroBurner]

I just found your PR, could you propose the switch upstream https://github.com/madler/zlib