cmake: make possible to build both game and engine against the engine Freetype submodule

[illwieckz]

Make possible to build both game and engine against the engine Freetype submodule.

Extracted from #1585:

• ci: use submodules for freetype #1585

Without the controversial change about default building.

See also:

• cmake: make possible to build both game and engine against the engine Freetype submodule Unvanquished/Unvanquished#3353

[slipher]

LGTM

[slipher]

Wait, what is FREETYPE_INTERNAL_ZLIB supposed to mean? We have the submodule configured so that there is no zlib dependency at all, right? The zlib dependency only comes in now for our poorly built Windows or Mac external_deps Freetype.

[illwieckz]

Freetype ships with an internal zlib, this is good enough for the nacl game because this is the only usage of zlib in the nacl game, so we use FREETYPE_INTERNAL_ZLIB, it prevents us to require a zlib submodule just to build the nacl game.

But then the engine may share zlib with freetype and libpng, so we don't use the internal freetype zlib to avoid having two zlib in the engine binary (they don't conflict as far as I know but it's just stupid to have zlib twice).

[illwieckz]

Hmm, rereading the code, we may use FREETYPE_INTERNAL_ZLIB only with NACL then.

[slipher]

We disable Brotli, bzip2, Harfbuzz, and PNG which means that we won't have a zlib dependency according to the comment on https://stackoverflow.com/a/29456684.

[illwieckz]

Yes but the engine itself links against libpng to open PNG images, so it already links against zlib.

Do you mean we completely avoid zlib when building Freetype? I'm afraid we cannot avoid that, unlike FT_DISABLE_BZIP2 that disables bzip2 entirely, FT_DISABLE_ZLIB switches to internal Freetype zlib. The Freetype cmake doesn't give options to disable ZLIB entirely.

[illwieckz]

The configure's --with-zlib=no option also doesn't allow to entirely disable zlib:

$ cd libs/freetype
$ ./autogen.sh
$ ./configure -h
…
 --with-zlib=[yes|no|auto]
                          use system zlib instead of internal library
                          [default=auto]
…

[illwieckz]

I added the commit to only use the Freetype internal zlib when building an NaCl game.

[slipher]

I read the Freetype CMakeLists.txt and it doesn't have internal zlib. I think it only implements a subset of the options of the configure script. When using CMake, if you disable zlib, there is really no zlib at all. make VERBOSE=1 freetype seems to confirm this.

[slipher]

I was wrong, it does implement internal zlib. I couldn't see it being built because it works by #including many .c source files. No, they couldn't just build it normally, gotta make hundreds of lines of patches to zlib so we can #include it!

There is an FT_CONFIG_OPTION_USE_ZLIB that we could theoretically use to turn off zlib. I guess we'd have to make our own customized ftoptions.h.

[slipher]

LGTM