make: do not delete autotools generated file with distclean

make: do not delete autotools generated file with distclean

                                        [...] running the command
`make maintainer-clean' should not delete `configure' even if
`configure' can be remade using a rule in the Makefile.  More
generally, `make maintainer-clean' should not delete anything that
needs to exist in order to run `configure' and then begin to build
the program.  This is the only exception; `maintainer-clean' should
delete everything else that can be rebuilt.