xapian-bridge C port

endlessm/eos-sdk#2463
This use of `TESTS_ENVIRONMENT` still works with the serial test runner, but is not officially supported. ([link](http://www.gnu.org/software/automake/manual/automake.html#Parallel-Test-Harness)). Instead use the syntax which is compatible with the parallel test runner:
```
TESTS_ENVIRONMENT = \
    export G_TEST_SRCDIR=$(abs_srcdir); \
    export G_TEST_BUILDDIR=$(abs_builddir); \
    $(NULL)
```
Specifically, `TESTS_ENVIRONMENT` must end with a semicolon.

Also, where are `G_TEST_SRCDIR` and `G_TEST_BUILDDIR` from? They sound like something that I ought to know about, but don't seem to be documented.