Updated README.

Added missing dependencies, links to dependency upstream homepages, and some
notes about testing.
Make everything buildable out-of-tree.
t/Makefile: Use top_srcdir rather than top_builddir.

This allows running tests out-of-tree.
src/Makefile: Fixed path to connection-private.h.
Makefile: Fixed generation of the ‘version’ file.
src/Makefile: Added a rule to create ‘../version’.
src/Makefile: Include sysdb.h.in file in a distribution.
.gitignore: Ignore test-driver.

This file is created by newer autotools when running ‘make check’.
t/Makefile: Pass gcov flags to tests as well.

Else, some tests might not compile correctly.
coverage.sh: Added a helper script to generate HTML coverage reports.

The script uses lcov and creates a clean copy of the source code in a
temporary directory using 'make dist'.
Makefile: Added coverage.sh to a dist tarball.