Add code coverage using lcov and gcov

To generate the code coverage report you need gcov and lcov installed
and then execute

 ./configure --enable-code-coverage
 make check-code-coverage

 Writing directory view page.
 Overall coverage rate:
   lines......: 76.5% (10519 of 13759 lines)
   functions..: 85.5% (491 of 574 functions)
   branches...: 57.5% (7040 of 12250 branches)
 file:///home/user/asn1c/asn1c-0.9.28-coverage/index.html

Open the index.html with a web browser
diff --git a/asn1c/tests/Makefile.am b/asn1c/tests/Makefile.am
index 15432fd..e6700c0 100644
--- a/asn1c/tests/Makefile.am
+++ b/asn1c/tests/Makefile.am
@@ -1,3 +1,4 @@
+@CODE_COVERAGE_RULES@
 
 SUBDIRS = check-src