Move gcc compiler option -Werror to CMAKE_CXX_FLAGS_DEBUG to work around
cmake using any options present in CMAKE_CXX_FLAGS as default options
for all build types.
This fix was necessary to prevent gcc from erroring out with indetifier
'requires' being a keyword sicne we are using c++11 and not c++20.
Coming from personal inconvenience - I've tried to build this from source, and was faced with Microsoft accounts not working. Upon re-reading the READMEs, I didn't find a reason for it, so I resorted to messaging in the Discord, which, after some back and forth, allowed me to understand that additional private stuff is needed for it to work. The question could've been avoided entirely, in my opinion, if this was included in BUILD.md - the place where anyone intending to build from source will come for guidance.
(Not) Secrets should've been linked clearly because it's a crucial piece of info for someone building from source, since normally. I'd expect the source code to fully reflect the pre-built binaries.
The Oracle JDK 8 is now offered at a commercial license and requires an account to download. OpenJDK has no such license and is completely free to use.
the `git clone` command provided doesn't work and gives this error on Debian
```
$ git clone --recursive git@github.com:MultiMC/MultiMC5.git src
Cloning into 'src'...
The authenticity of host 'github.com (192.30.253.113)' can't be established.
RSA key fingerprint is SHA256:nThbg6kXUpJWGl7E1IGOCspRomTxdCARLviKw6E5SY8.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'github.com,192.30.253.113' (RSA) to the list of known hosts.
git@github.com: Permission denied (publickey).
fatal: Could not read from remote repository.
Please make sure you have the correct access rights
and the repository exists.
```
I am suggesting an alternative command to provide to users in BUILD.md instead.
