Cannot compile with SQ_DISABLE_INSTALLER

Compiling with SQ_DISABLE_INSTALLER activated results in a CMake failure.

Steps to reproduce:

In a folder build inside the repository, run cmake -DSQ_DISABLE_INSTALLER=ON ... This will give the following error:
```
CMake Error at CMakeLists.txt:90 (export):
export Export set "squirrel" not found.
```

I'm using Linux + CMake 3.16.3.

Note: commenting lines 90 to 93 in CMakeLists.txt fixes the issue; perhaps an if(NOT SQ_DISABLE_INSTALLER) is missing?

albertodemichelis / squirrel