fixup: avoid unconditional re-bootstrapping on "make dist"

fixup: avoid unconditional re-bootstrapping on "make dist"

Since our 'bootstrap' script is distributed, the 'distdir' target depends
on it.  But in our GNUmakefile, we also have a 'bootstrap' target declared
.PHONY, which when called re-bootstrap the Automake distribution.  Thus,
whenever we run "make dist", GNU make sees it must remake the 'bootstrap'
target (as it is a dependency of the 'distdir' target), and thus ends up
re-bootstrapping all the package (because of the .PHONY 'bootstrap' target
in GNUmakefile).

We fix this issue by renaming our bootstrap script to 'bootstrap.sh'.

* bootstrap: Renamed ...
* bootstrap.sh: ... to this.
* GNUmakefile (bootstrap): Adjust.
* Makefile.am (EXTRA_DIST): Likewise.
(autodiffs): Likewise.
* HACKING: Likewise.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>