This commit alters the build system substantially; it adds another
platform, `headless`, that provides stubs in place of all GUI
functions, and provides a library `solvespace_headless` alongside
the main executable. To cut down build times, only the few files
that have #if defined(HEADLESS) are built twice for the executable
and the library; the rest is grouped into a new `solvespace_cad`
library. It is not usable on its own and just serves for grouping.
This commit also gates the tests behind a -DENABLE_TESTS=ON CMake
option, ON by default (but suggested as OFF in the README so that
people don't ever have to install cairo to build the executable.)
The tests introduced in this commit are (so far) rudimentary,
although functional, and they serve as a stepping point towards
introducing coverage analysis.
After this change, SolveSpace does not contain nonfree assets.
Additionally, Perl is not required for the build.
Note that in the US, case law suggests that copyright does
not apply to bitmap fonts:
http://www.renpy.org/wiki/renpy/misc/Bitmap_Fonts_and_Copyright
Nevertheless, it was prudent to replace the asset with something
that is unambiguously free.
As a side effect, zlib and libpng are now git submodules,
based on their respective official git repositories.
This is necessary, because MinGW has a different ABI and
it cannot use the prebuilt binaries built by MSVC.
The submodules are also used for Windows, for several reasons:
* to allow 64-bit builds;
* to allow using newer MSVC, which doesn't like the prebuilt
libraries;
* to keep the libraries updated.
whitequark