check: support colorized testsuite output by default

check: support colorized testsuite output by default

Fixes automake bug#11855.

With this change, we make the 'color-tests' option enabled by default.
That option remains still silently accepted as a no-op, for backward
compatibility.

The developer of a package is still free to disable testsuite coloring on
a per-makefile basis (by adding "AM_COLOR_TESTS = no" to the Makefile.am)
or on a whole-project basis (by AC_SUBST'ing AM_COLOR_TESTS to "no" in
configure.ac).  But now the user will be able to request the testsuite
output to be colorized, if he really wants to:

    # With GNU make:
    make AM_COLOR_TESTS=yes check
    # With non-GNU make:
    make AM_COLOR_TESTS=yes AM_MAKEFLAGS=AM_COLOR_TESTS=yes check

* NEWS: Update.
* doc/automake.texi: Updated, and some related minor reformatting
and rewording.
* automake.in (handle_tests): No need to pass the transform '%COLOR%'
when processing "check.am".
* lib/am/parallel-tests.am: Remove use of the '%?COLOR%' transform;
just act as if it were unconditionally TRUE.
* t/color.sh: No need to specify 'color-tests' in 'AUTOMAKE_OPTIONS'
nor in 'AM_INIT_AUTOMAKE'.
* t/ax/testsuite-summary-checks.sh: Likewise.
* t/ax/tap-summary-aux.sh: Likewise.
* t/color2.sh: Likewise.  Also, ensure that colorized testsuite output
can be disabled by default by calling "AC_SUBST([AM_COLOR_TESTS], [no])".
* t/tap-realtime.sh: Define 'AM_COLOR_TESTS' to "no" in Makefile.am, to
avoid spurious colorization of the output due to the use of the 'expect'
program.
* t/color-tests-opt.sh: New test, check that the 'color-tests' option is
still recognized as a no-op.
* t/list-of-tests.mk: Add the new test.

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