[Bug 770987] configure.ac EGL checks clobber user EGL_LIBS/EGL_CFLAGS

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=770987

--- Comment #6 from Matthew Waters (ystreet00) <ystreet00 at gmail.com> ---
(In reply to Alistair Buxton from comment #5)
> I don't think this is fixable, or if it is then it is beyond my ability.
> 
> The problem is basically that when the RPi check runs, EGL_CFLAGS/LIBS can
> contain one of three different things:
> 
> 1. Flags from pkg-config.
> 2. Flags from the user.
> 3. Nothing at all.

IIRC, pkg-config will not overwrite the EGL flags (or any other flags) if they
are set already.  We just need to do the same.

> Additionally the values can:
> 
> A. Work.
> B. Work, but point to the wrong EGL implementation.
> C. Not work at all.
> 
> So what the check does is completely ignore EGL_CFLAGS/LIBS and test
> directly for the Broadcom libs. If it finds them then it throws away any
> existing value and rewrites the variables from scratch. Thus preventing
> pkg-config from picking the wrong implementation.

This problem can be "fixed" I think by moving the pkg-config detection after
the rpi/mali specific detection as pkg-config won't override user set
variables.

> But where this fails is when you have the libs outside the system path. Then
> you have to use the global LIBS and CPPFLAGS (because LDFLAGS and CFLAGS
> won't actually work) - because the check ignores EGL_* and then if it
> successeds it overwrites them with incomplete hard coded information.
> 
> I could fix it so it doesn't ignore user supplied values, but then it will
> probably break for someone else.

This is the correct fix and should be completed.

> I did implement GST_GL_* but it doesn't actually change anything for me.

This is mostly for the GL side so we don't override pkg-config/user's choice
for the opengl library (not gles2).

> I also found a couple of other bugs while doing that.

Patches! patches! everywhere.

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