sdl2-sys: Avoid to add unwanted system folder in library research paths

[rofferom]

Default pkg-config-rs behavior is to add default system library paths (ex:
-L/usr/lib/x86_64-linux-gnu).

This is because PKG_CONFIG_ALLOW_SYSTEM_LIBS is set by pkg-config-rs by
default. print_system_libs(false) disable this behavior.

More details in evdev-rs project:
ndesh26/evdev-rs#85

[Cobrand]

I'm sorry it seems i didn't have the notification a few months ago.

Isn't partially the point of pkg-config to use system libs and find where they are? If you disable that, won't pkg config fail to find the sdl2 paths? I'm having a hard time understanding what would happen to the average user

[rofferom]

Actually, gcc has a default research path for system libraries. For the average user, my patch should have no impact at all. On my computer, I get correct flags with :

$ pkg-config --cflags --libs sdl2
-D_REENTRANT -I/usr/include/SDL2 -lSDL2

Instead, the default behavior of Rust pkg-config crate is

$ PKG_CONFIG_ALLOW_SYSTEM_LIBS=1 pkg-config --cflags --libs sdl2
-D_REENTRANT -I/usr/include/SDL2 -L/usr/lib/x86_64-linux-gnu -lSDL2

A redundant -L option is added. Like I mentioned in ndesh26/evdev-rs#85, it is only an issue if you want yo use a custom version of a library in your project.

I think there is no impact for your project, but I would understand if you don't agree with that.

[Cobrand]

Actually, gcc has a default research path for system libraries.

I don't think is this true for static linking, only for dynamic linking. I know the pkg-config feature was introduced partially due to static linking. Could you please test that it still works with your PR?

[rofferom]

Actually, gcc has a default research path for system libraries.

This is "true", but we are not using gcc. I made some tests to force a sample C program to statically link with SDL2, there is no issue when system. But when I'm forcing a static link when using rust-sdl2, I have a link issue. So my initial statement is false in the context of this PR :)

I don't think is this true for static linking, only for dynamic linking. I know the pkg-config feature was introduced partially due to static linking

You are right rust-lang/pkg-config-rs#11, there are many complains link mine, but what you said is explained many times. There are also many references of projects that are doing the same kind of change than this PR.

Maybe the correct update of my patch would be

-        .print_system_libs(false)
+        .print_system_libs(statik)