Fix bug in linking tests.

The existing code was only building object files.
Add a phony 'tests' rule.

This rule builds all tests.
Recommend 'make -r' invocation in README.

The builtin rules of 'make' are of no use when applied to the generated
makefiles, and can sometimes cause issues.
Add an example advertising the 'tests' target.
No hardcoding of cflags/ldflags.

Prior to this commit, '-Dunix' was being hardcoded as a cflag for generated
ninja build files in order for 'bsl' to build on 'OS X'.  This is no longer the
case - users may specify their cflags and ldflags via command line options to
the generator subcommands.
Remove spurious library in test ldflags.

Prior to this commit, a test belonging to a package group had itself
incorrectly added to the test's ldflags.