If Mesa is built without X11 headers, building against Mesa EGL headers
requires a dependency on egl.pc, to define MESA_EGL_NO_X11_HEADERS.
This fixes a build error when compiling ext/qt/gstqtglutility.cc:
In file included from /usr/include/EGL/egl.h:39,
from /usr/include/gstreamer-1.0/gst/gl/egl/gstegl.h:44,
from ../gst-plugins-good-1.16.1/ext/qt/gstqtglutility.cc:43:
/usr/include/EGL/eglplatform.h:124:10: fatal error: X11/Xlib.h: No such file or directory
On other OSes, it's not possible to have qmake or the qt5 pkg-config
files and not have moc, and `moc` will not be in `PATH`, so this only
causes problems.
Needs gnustl for C++ STL support, which is the GNU STL on Android API
19 and older, and is a wrapper for the llvm-libc++ STL on newer APIs.
QtGui C++ templates use GL functions, so GLESv2 is needed at link time
The android code path is slightly different than the EGLFS one,
so I added previously a HAVE_QT_ANDROID define for use with qmake.
Here I also add it in meson, although I expect nobody will ever use
meson to build this, as it's complicated.
Tested on linux with X11/wayland and semi-tested on Windows.
Windows crashes on item destruction however this is better than nothing.
Fix up some win32 build issues on the way with mismatched {} and
G_STMT_{START,END}
