CMakeLists.txt: explicitly declare LONG_OUTPUT_NAMES and DISABLE_STATIC
CMakeLists.txt: explicitly declare LONG_OUTPUT_NAMES and DISABLE_STATIC

Toralf noticed a minor infelicity in option declarations:
```
build $ cmake -DLONG_OUTPUT_NAMES=YES ..
CMake Warning:
  Manually-specified variables were not used by the project:

    LONG_OUTPUT_NAMES
```

Note, CMake complains here about "unused" LONG_OUTPUT_NAMES.
The option is used but not declares as user-tweakable via 'option()'.

This change adds declarations for variables used in Gentoo.

Reported-by: Toralf Förster
Bug: https://bugs.gentoo.org/659880
Signed-off-by: Sergei Trofimovich <110fdadc76b6bce16977f568bdb4723eaed6fb3f@gentoo.org>

albertodemichelis/squirrel,albertodemichelis/squirrel,mfxmfx/squirrel,mfxmfx/squirrel