Add code coverage support

The coverage report shows what code is covered by tests and what is not
and the ratio could be tracked over time. These reports will allow us
to identify code that is not being tested and improve the test suites.

To enable the reports configure with --enable-code-coverage and execute
"make check-code-coverage". The HTML report will be generated in a
subdirectory with name libosmocore-$(PACKAGE_VERSION)-coverage/index.html

The report is generated using gcov, lcov and lcov_cobertura tools and
the OSMO_AC_CODE_COVERAGE macro. The osmo_ax_code_coverage.m4 is a copy of
ax_code_coverage.m4 taken from autoconf-archive v2018.03.13. It was
copied to avoid the additional external dependency and renamed to avoid
overwriting it in case autoconf-archive is already installed as we are
going to install it in $(datadir)/aclocal in order to be reused in other
osmocom's projects.

Closes: OS#1987
Change-Id: I6f4ffb91bd7f3dd070aa09dd16d5ad1faf130a4c
diff --git a/configure.ac b/configure.ac
index 6c54e66..060d081 100644
--- a/configure.ac
+++ b/configure.ac
@@ -357,6 +357,8 @@
 	AM_CONDITIONAL(HAVE_SSE4_1, false)
 fi
 
+OSMO_AC_CODE_COVERAGE
+
 dnl Check if the compiler supports specified GCC's built-in function
 AC_DEFUN([CHECK_BUILTIN_SUPPORT], [
   AC_CACHE_CHECK(