check: Have autotools generate internal-check.h

check: Have autotools generate internal-check.h

Previously GStreamer got access to the libcheck interface by including
libs/gst/check/check.h which in turn included internal-check.h in the
same directory. internal-check.h was generated by copying
libs/gst/check/libcheck/check.h which in turn was generated from
check.h.in in the same directory. In this case generating
libs/gst/check/libcheck/check.h is unnecessary, in addition this file
was accidentally distributed in generated project tarballs.

Now libs/gst/check/internal-check.h is generated directly from
libs/gst/check/libcheck/check.h.in by configure. This means that the
libcheck source must include internal-check.h instead of the previously
generated libs/gst/check/libcheck/check.h. However the unnecessary
intermediate step is now skipped.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=741359