autogen.sh: Only copy submodules if building from a git branch.

`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 that various git commands are only called and 'dlg' files
are only copied if `autogen.sh` is called from a git repository.

autogen.sh

@@ -182,17 +182,19 @@ copy_submodule_files ()
   cp $DLG_SRC_DIR/* src/dlg
 }
 
-DLG_INC_DIR=subprojects/dlg/include/dlg
-DLG_SRC_DIR=subprojects/dlg/src/dlg
+if test -d ".git"; then
+  DLG_INC_DIR=subprojects/dlg/include/dlg
+  DLG_SRC_DIR=subprojects/dlg/src/dlg
 
-if test -d "$DLG_INC_DIR"; then
-  :
-else
-  echo "Checking out submodule in \`subprojects/dlg':"
-  git submodule init
-  git submodule update
-fi
+  if test -d "$DLG_INC_DIR"; then
+    :
+  else
+    echo "Checking out submodule in \`subprojects/dlg':"
+    git submodule init
+    git submodule update
+  fi
 
-copy_submodule_files
+  copy_submodule_files
+fi
 
 # EOF