I don't know if we're planning to have this ever be turned off, but at
least the ambiguity between SDL & SDL_GFX is removed.

Also, USE_SDLGFX is renamed to USE_SDL_GFX, to fit CMake's style. (Not
worrying too much since currently -DUSE_SDLGFX doesn't build anyways.)

I have barely tested this, but it's better than no builds to test?

Also, fun fact, launching this build on a fresh user data directory
(no xm.db) corrupts the database because the version doesn't ever get
written! Or something. I had to remove it anyhow while testing.

This module was added in CMake 3.14, but we can vendor it until the next
Debian stable release.

If the build is using system libraries (for bzip2, lua, ode, or
xdgbasedir), there's no need to add the corresponding subdirectories.
This reduces build time and avoids potential mix-ups.

Signed-off-by: Stephen Kitt <steve@sk2.org>

This commit switches over the following:
+ CURL (library, REQUIRED)
+ JPEG (library, REQUIRED)
+ Xml2 (library, REQUIRED)
+ PNG (library, REQUIRED)
+ SQLite3 (library, REQUIRED)
+ ZLIB (library, REQUIRED)

This commit switches over the following:
+ Intl (library, if(USE_GETTEXT) -> REQUIRED)
+ LibLZMA (libary, if(STATIC_BUILD))
+ SDL (library, if(USE_SDL) -> REQUIRED)
+ SDL_gfx (library, if(USE_SDL) -> if(USE_SDL_gfx) -> REQUIRED)
+ SDL_mixer (library, if(USE_SDL) -> REQUIRED)
+ SDL_net (library, if(USE_SDL) -> REQUIRED)
+ SDL_ttf (library, if(USE_SDL) -> REQUIRED)

This commit switches over the following:
+ OpenGL (library, if(USE_OPENGL) -> REQUIRED)

This commit switches over the following:
+ BZip2 (library, system/vendor)
+ Lua (library, system/vendor)
+ ODE (library, system/vendor)
+ XDG (library, system/vendor)

Additionally, the following are brought into style:
+ Chipmunk (library, vendor)
+ Md5sum (library, vendor)
+ Glad (generated) (library, vendor)

This only worked on my system because of `if(NOT Intl_LIBRARY)`.

X-Moto allows certain libraries to be provided either by the system or
X-Moto's source tree itself, as vendored dependencies. (Not all vendored
dependencies may be provided by the system, such as Chipmunk.) These are
called vendorable libraries here.

With a USE_SYSTEM_* option ON, that vendorable library is system. With a
USE_SYSTEM_* option OFF, that vendorable library may be either system or
vendored.

With the USE_SYSTEM option OFF, all USE_SYSTEM_* options are OFF.
With the USE_SYSTEM option ON, all USE_SYSTEM_* options default ON.

With a PREFER_SYSTEM_* option ON, that vendorable library is searched
for and is system if found.

With the PREFER_SYSTEM option ON, all PREFER_SYSTEM_* options default
ON. With the PREFER_SYSTEM option OFF, all PREFER_SYSTEM_* options are
OFF.

With the USE_SYSTEM option OFF, the PREFER_SYSTEM option defaults ON.
With the USE_SYSETM option ON, the PREFER_SYSTEM option is OFF.

The upshot of all this is that USE_SYSTEM ON means libraries are
explicitly either system or vendored, with failed system searches
erroring instead of falling back to vendored, and both USE_SYSTEM &
PREFER_SYSTEM OFF means all libraries are vendored.

This should be good for maintainers?

Yes:

    #include <lua.h>

No:

    #include <lua/lua.h>

https://github.com/travis-ci/travis-build/blob/c27f0b048a965a6824c3b31d731df7617b185b63/lib/travis/build/addons/apt.rb

https://github.com/travis-ci/travis-build/blob/c27f0b048a965a6824c3b31d731df7617b185b63/lib/travis/build/addons/homebrew.rb

wheee Travis has CMake 3.12.4