.gitignore: Ignore test-driver (generated by autoreconf)
Specify LT_INIT in configure.ac

* configure.ac (LT_INIT): Specify (otherwise Automake 1.13.4 reports
error at "check_LTLIBRARIES" item in Makefile.am); add comment about
"LIBTOOL is undefined" error.
Remove configure.ac outdated revision number

* configure.ac (AC_REVISION): Remove item (outdated).
Replace deprecated [CXX]INCLUDES to AM_C[PP]FLAGS in configure.ac

* configure.ac (INCLUDES): Replace with AM_CFLAGS (as the former is
deprecated in Automake 1.13.4).
* configure.ac (CXXINCLUDES): Replace with AM_CPPFLAGS.
Prevent misleading AC_MSG_ERROR/AS_IF errors reported in configure.ac

* configure.ac (PKG_CHECK_MODULES): Move AS_IF and AC_MSG_ERROR out of
ACTION-IF-NOT-FOUND argument (setting "missing_libatomic_ops" variable
instead) to prevent misleading
"possibly undefined macro: AC_MSG_ERROR, AS_IF" error reported by
Autoconf (pointing to code lines where these macros first occur) in
case of PKG_CHECK_MODULES is not defined in aclocal.m4; add comment
about missing Autotools pkg.m4 file.
tests: Append _test suffix to 'initsecondarythread' binary file names

* tests/tests.am (TESTS, check_PROGRAMS, initsecondarythread_SOURCES,
initsecondarythread_LDADD): Rename initsecondarythread to
initsecondarythread_test (except for .c file).
* .gitignore: Likewise.
Rename subthread_create to subthreadcreate_test (Makefile)

* tests/tests.am (TESTS, check_PROGRAMS, subthreadcreate_SOURCES,
subthreadcreate_test_LDADD): Rename "subthread_create" to
"subthreadcreate_test" (except for .c file).
* .gitignore: Likewise.
.gitignore: Ignore *.log, *.trs files produced by 'make check' (Autoconf)