The out-of-source build syntax for gcovr 4.2 is different compared to
previous versions and therefore an update was needed. In researching the
most appropriate solution it was found that any gcovr version older than
3.3 always resulted in 0% coverage. Because of this, rather than adding
an additional layer of logic, some already existing logic was modified
to ensure correct syntax for the new version, while versions older than
3.3 are flagged as not supported.

Closes mesonbuild#5089.

The code our projects care about verifying coverage for mostly lives in
the source_root with the exception of the generated source files in
build_root. This change cleans up the output so we don't have prefixed
paths on our source files anymore.

summary from stdout is often used by Automated builds to show build details

The --print-summary option was added to gcovr in v3.2, since html output
was added only in 3.1, limitting support to 3.2 won’t be a big deal.

--print-summary is not enabled for text/xml report generation as it will
result in meson not supporting any gcovr version less than 3.2.

lcov branch coverage are disabled by default.  So branch
coverage has to be enabled for every stage to make them work.

Fixes https://github.com/mesonbuild/meson/issues/4319

Restore subproject exclusion for the html coverage report that existed
in the ninja backend legacy target.

Also exclude subprojects for the gcovr generated reports.

ninja coverage -> generate all possible reports (text, xml, html)
                  depending on gcovr and/or lcov/genhtml availability.
ninja coverage-html -> generate only html report
ninja coverage-xml -> generate only xml report
ninja coverage-text -> generate only text report

Make all targets phony, the old legacy rules where just annoying as
you would have to remove the old report before being able to generate
a new one.

ninja coverage succeeds if it can generate at least one report.
ninja coverage-* only succeeds if it can generate the requested report

Rename gcovr_3_1 to gcovr_new_rootdir