cmake: do not install the static libraries by default

[illwieckz]

Add the INSTALL_STATIC_LIBS option to allow the installation (or not) of static libraries.

Installing static libraries is useful in the context of a distribution package of libcrn itself.

But for projects linking against libcrn for there own package, it is useless to distribute it.

This has been motivated by me using libcrn to add CRN support in the XQF server browser:

• Implement CRN/DDS/KTX levelshot loading XQF/xqf#239

[illwieckz]

This looks obvious enough.

[slipher]

If you're including the crunch cmake into a super-project wouldn't you want to name it like CRUNCH_xxx? Naming it just INSTALL_STATIC_LIBS is confusing because one would assume it's for the top-level project.

[illwieckz]

Or INSTALL_STATIC_LIBCRN since we already have BUILD_STATIC_LIBCRN.

[illwieckz]

Now renamed as INSTALL_STATIC_LIBCRN.

[illwieckz]

Since it's OFF by default we can rename it as we like anyway, projects including the Crunch CMakeLists.txt file and not wanting to install the static lib just have nothing to set.