Introduce a new tests/conv.c source file which exercises the endianess
conversion macros. It's assumed that some use cases may break their
operation, fortunately these edge cases were not seen before, or the
unreliable operation went unnoticed. This test raises awareness of the
implementation's constraints.
This is a start, the test sequence will benefit from adding some more
cases to increase coverage.
Since Autoconf places some important feature flags only into the
configuration header, it is necessary to include it globally to
guarantee a consistent build.