Edit .gitignore to ignore additional files.

The files being ignore include :
* ./configure auto generated file
* compilation result
Edit autogen.sh and add remove-autogen.sh

Old autogen.sh isn't work on some old system. The fix is to create m4/ directory before execute autoreconf.
Also, to make sure clean tree, remove-autogen.sh is added to make tree cleaning easier. The script will execute "make distclean" before remove all files generated by autogen.sh