Correct license block in src/timing.c

The license block in src/timing.c suggested that it was not
GPL-licensed.  That is, however, incorrect; since the release of
ppamltracer, src/timing.c has been licensed under the GPLv3+.  The
license block is now corrected.
Functional tests for C library

While gearing up to make an addition to the C library, I became
concerned at the lack of test infrastructure for the project.  The build
system now integrates Check <http://check.sourceforge.net/>, a
lightweight testing framework for C, and there are a few tests to ensure
no serious regressions occur.  You can run the tests with “make check”.