Cope with whitespace in $MISSING and $install_sh.

Cope with whitespace in $MISSING and $install_sh.

* configure.ac (am_AUTOHEADER): New substitution, save the value
of $AUTOHEADER before AM_INIT_AUTOMAKE may add $MISSING.
* tests/defs.in: Use am_AUTOHEADER.
* lib/am/install.am: Fix typo.
* m4/install-sh.m4 (AM_PROG_INSTALL_SH): Add suitable
single-quote quoting to install_sh, but only if needed.
* m4/missing.m4 (AM_MISSING_HAS_RUN): Add suitable double-quote
quoting to MISSING, but only if needed.
* m4/sanity.m4 (AM_SANITY_CHECK): Abort configure if `pwd` or
$srcdir contain shell meta-characters that cannot be handled;
space and tab are allowed in the former only.
* tests/sanity.test: New test.
* tests/Makefile.am: Adjust.
* NEWS: Update.
Reports by Jim Meyering and others.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>