Add option "-src-dir <source directory>" to build options.
When calling build.sh without -src-dir it uses src/subsurface as usual, but when called
with -src-dir <source directory>, it uses src/<source directory> as source basis.
This is a needed option, when working with "git worktree", which is used when working on
different branches in parallel (e.g. master and my-feature-branch), because it allows a
build directory in each worktree, and thus much faster when switching work.
Signed-off-by: jan Iversen <jan@casacondor.com>
20a1b4befb