Fix conditional build wrt zip archives and cx11

Fix conditional build wrt zip archives and cx11

Revert 1b4e3844e959c8261ba5cab9a0c5c34bcb812344. Automake does support
nested conditionals just fine.  One just has to use the += operator.

This also fixes the problem:

	./configure: line 15878: ENABLE_ZIP_ARCHIVE_AND_CXX11:
	command not found

caused by bad syntax:

	ENABLE_ZIP_ARCHIVE_AND_CXX11 = yes

(Variable assignments must not use spaces around '=').

	* configure.ac: remove (broken) assignment to the
	ENABLE_ZIP_ARCHIVE_AND_CXX11 variable
	* Makefile.am: replace variable assignments to
	ZIP_ARCHIVE_TESTS_FIRST_PART, ZIP_ARCHIVE_TESTS_SECOND_PART
	by ZIP_ARCHIVE_TESTS using the "+=" automake operator.
	Likewise for CXX11_TESTS.

Signed-off-by: Jan Engelhardt <jengelh@inai.de>
Signed-off-by: Dodji Seketeli <dodji@redhat.com>