Use AX_CODE_COVERAGE for test-coverage statistics

Use AX_CODE_COVERAGE for test-coverage statistics

DBus uses custom rules in its Makefiles to implement test-coverage
statistics.

This patch implements test-coverage statistics with the autoconf macro
AX_CODE_COVERAGE. The script automatically tests for tools (e.g., gcov,
lcov), sets build variables and creates Makefile rules.

Run 'configure' with '--enable-code-coverage' to enable support for
test-coverage statistics. Run 'make check-code-coverage' to run the
tests and generate the statistics.

Signed-off-by: Thomas Zimmermann <tdz@users.sourceforge.net>
[smcv: do not alter compiler.m4; move AM_CXXFLAGS to the one place we
compile C++]
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=88922