my/m4/ax_code_coverage.m4: new

from http://savannah.gnu.org/patch/?8451
my/m4/ax_code_coverage.m4 -> my/my_code_coverage.m4
my/m4/my_code_coverage.m4: remove lcov version check
my/m4/my_code_coverage.m4: set CODE_COVERAGE_DIRECTORY to $(abs_top_builddir) and use it consistently