Generate coverage information.

* autogen.sh: add options --coverage and --report to help w/ coverage
info collection and reporting.
* m4/gcov.m4: new file. Boilerplate for locating gcov et.al.
* Makefile.am: add coverage-report target

GnuPG-Bud-Id: 3050

To measure test coverage gcov and lcov needs to be installed and
./configure called w/ --enable-gcov. The coverage-html make target will
then create a HTML report.