This commit adds a new default build system for ipcalc, marking the
existing GNU Makefile as "legacy", but not removing it entirely (at
least not yet). The main benefits of Meson for this project are that it
will automatically detect the presence of the maxminddb and geoip
libraries on the system and use the best available one by default rather
than just assuming that maxminddb is always available by default like
the Makefile does, and that it makes cross-builds easier, particularly
with dependencies. Normally a key benefit of Meson is that the configure
times and build times are significantly reduced compared to most other
build systems (with the possible exception of CMake with the Ninja
backend, according to the official Meson documentation), but while that
is technically true for this project as well, since ipcalc is such a
relatively small application, the reduced build time is fairly
negligible.

This commit also restructures the ipcalc unit tests, defining them all
in one file (tests/meson.build) and grouping them by the feature that
they are testing to hopefully make them a little bit easier to maintain.
This could have been done with the old Makefile instead - nothing about
Meson is technically required for the restructured unit tests - but it
made sense to do while introducing the new Meson build system for ipcalc
because it allows the 'meson test` utility to differentiate between
individual tests, summarize the passed/failed test results, run all
tests in parallel (making a full test run nearly instantaneous), and
allow a developer to run only specific tests while debugging.

Signed-off-by: Karl Lenz <xorangekiller@gmail.com>