autogen.sh: Only copy submodules if building from a git repository
autogen.sh fails if building from a standard source tarball. Firstly, git expects to be called in a git repository, then copy_submodule_files() blindly attempts to copy files.
Debian, Ubuntu, Linux Mint and other derivatives all run autogen.sh before compiling to regenerate build files.
This patch ensures variou git commands and dlg files are only copied if autogen.sh is called from a git repository.
Edited by Hugh McMaster