configure.ac: fix OSX host check

Makes the check works with (i386|x86_64)-apple-darwin* hosts.

https://bugzilla.gnome.org/show_bug.cgi?id=727104
gl: only set CAPS_FEATURE_MEMORY_EGL_IMAGE on egl platform
gl: fix out-of-source builds pt2

21b897de2f762bee504cbc007fd108148ee5ae24 did this for x11 and wayland.
Do it for the other backends as well.
gl: don't fail configure if we do not understand the platform
eglglessink: fix wrong doc block for create_window

Wrong since 1ad253b9 when prop name was changed from
can_create_window to create_window.
eglglessink: unref last buffer on DRAIN

Similar to 093574053fae243ec5fcfd6adae8185053b1e896
in gstbasesink

https://bugzilla.gnome.org/show_bug.cgi?id=726107
gl/win32: allow not building the gl plugins

Fixes build on windows if <GL/wglext.h> headers are not present.

https://bugzilla.gnome.org/show_bug.cgi?id=727125
tests: fix build of gl unit tests in uninstalled setup

Put local .la files first, and add link to libgstvideo
for tests that use the libgstvideo API directly.
tests: don't link codecparser tests twice to the lib
tests: fix LDADD order for insertbin test
build: Fix make dist
gl: egl: fix distcheck and out of source build
hlsdemux: Implement parsing of #EXT-X-I-FRAME-STREAM-INF

These are I-frame-only variant lists that can be used
for trick mode playback.
hlsdemux: Implement trick modes via I-frame variant lists
hlsdemux: Send flush start event before waiting for the tasks to finish

Otherwise we'll wait until buffers are completely processed downstream,
which might take quite some time.
hlsdemux: Unset DTS of all buffers

We won't get a valid DTS from the source.
hlsdemux: Fix forwards and backwards searching in the files list
hlsdemux: Set DISCONT flag on all buffers in reverse playback mode
hlsdemux: Don't set the segment offset

It's causing wrong running times after seeks or bitrate
switches.
mpegtsbase: Fix pull mode scanning for PCR on small files

If a file does not contain 5 PCRs until it is EOS, or does not
contain more than 655360 bytes the PCR scanning algorithm just
aborted.