[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 #5 from Alistair Buxton <a.j.buxton at gmail.com> ---
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.

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.

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.

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

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

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