Do not remove distributed configure script

Do not remove distributed configure script

Before this patch the clean target has removed the
configure script that comes with Git tar file.
That made compiling Git for different architectures
inconvenient.
This patch excludes configure from the files to be
deleted by 'make clean' and adds new target 'distclean'
to preserve old functionality.

Signed-off-by: Mathias Megyei <mathias@mnet-mail.de>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>