misc: Add .gitignore file
The git status -u
shows the whole content of the 'build' directory
and the the top directories of the subprojects
directory, making
it too easy to overlook some files not being part of the git checkout,
when they are supposed to be there.
The proposed file contains three variants of the common 'build' directories and the other things, which helps to make the git status -u
clean.