Allow the Ninja build plan generated by configure.py to
build `ninja_test` by compiling GoogleTest from source if
the path to the library if passed through the new option
`--gtest-source-dir` or the GTEST_SOURCE_DIR environment
variable.

For simplicity, probing for an installed version of the
library, and linking to it, is not supported (use the
CMake build for this).

This also removes the obsolete `--gtest-dir` option.

+ Update README.md

Fixes #2447