Add configure --enable-code-coverage option

Add configure --enable-code-coverage option

* m4/ax_code_coverage.m4: New file.
* configure.ac: Use AX_CODE_COVERAGE.
* Makefile.am: Add @CODE_COVERAGE_RULES@.
(CODE_COVERAGE_LCOV_OPTIONS, CODE_COVERAGE_GENHTML_OPTIONS,
CODE_COVERAGE_IGNORE_PATTERN): New variables.
(strace_CPPFLAGS): Add $(CODE_COVERAGE_CPPFLAGS).
(strace_CFLAGS): Add $(CODE_COVERAGE_CFLAGS).
(strace_LDADD): Add $(CODE_COVERAGE_LDFLAGS).
* .gitignore: Add *.gcda and *.gcno.

Signed-off-by: Gabriel Laskar <gabriel@lse.epita.fr>
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>