Fix CMakeLists.txt error

[4G3NT]

Don't read CMAKE_BUILD_TYPE if it's not defined in order to fix an error:

CMake Error at CMakeLists.txt:75 (string):
string no output variable specified

-- Configuring incomplete, errors occurred!

[4G3NT]

We can also have CMAKE_PREFIX_PATH default to a valid value, but since the cmake file spits out an understandable error, I don't think that's necessary.

[Lymphatus]

For CMAKE_PREFIX_PATH it's a bit harder to have a default value. It varies upon platforms and you can always customize the Qt path for your specific setup. If you give a default value and that is not correct, the error that will come out is far harder to understand than the one that's in place right now.

[4G3NT]

Yeah, good point it's really hard to set it to a good default.

[4G3NT]

Second if statement might be unnecessary too.

[Lymphatus]

I'm no cmake expert whatsover and the current file is a bit of a mess, result of many trials and errors in the past. It works now, but I'd like to refactor it a little bit to make it more clear and useful in the future. It also helps that now Github Actions are in place and there's an automated way to know if a script works on all platforms at once.

[4G3NT]

I will delete the if statement as it is unnecessary and I will add another fix for the INSTALL make target.