[RFC] CMake: Set custom RelWithDebInfo compile flags.

Follow-up from #1366. Tested that it uses the correct flags with something like:

```
$ export PATH=$HOME/bin:$PATH
$ cat > $HOME/bin/cc
#!/bin/bash
echo "$*" >> $HOME/debugcc
/usr/bin/gcc $*
$ make CMAKE_BUILD_TYPE=NvimReleaseWithDebInfob
$ less $HOME/debugcc
After looking at the CMake source, I don't believe setting the DEBUG variable does anything for us.  The build type is what decides whether `-g` is added or not.  It is getting set as a flag in our config.h though.  So I'm not sure we really want RelWithDebInfo to trigger that.