First step in adding UWP/WinRT support to recipes

commit fc8b9456:

glib.recipe: Add patches for WinRT/UWP compatibility
Upstreaming these is going to be a long-ish road, so put the patches
in a subdir so it doesn't bother people.

commit 9d163694:

recipes/custom.py: Import FatalError
We use it but don't import it.

commit fb021184:

gst-plugins-base-1.0.recipe: Misc trivial fixes
libxml2 is not actually used, and the opengl plugin is built on all
platforms so there's no need to make platform-specific files
properties for it.

commit 972c9df6:

zlib.recipe: config.h defines can be 1 or 0 too

commit 593c35f4:

gst-plugins-good-1.0.recipe: gdk-pixbuf can be re-enabled on MSVC
The CRT interop issues have been resolved ever since we updated the
toolchain.

commit 6ae74e55:

gst-plugins-good-1.0.recipe: Explicitly disable waveform plugin
It was never shipped with our binaries. We were building it and
throwing it away.

commit 8e21ce52:

gst-plugins-bad-1.0.recipe: Misc trivial fixes
Enable dash plugin explicitly, and add the dependency that it was
missing. Also enable ladspa plugin explicitly, which was missed while
adding it.

commit 6f1d8ce5:

recipes/custom.py: Give a better error when plugin not found

commit b1b4a924:

windows.config: Add UWP flags to C++ flags too
Really silly oversight.

commit def1c586:

recipes/custom.py: Also remove library include dirs
Sometimes we install things into lib/gstreamer-1.0/include, so remove
those too when disabling a library.

With these changes, you can run -v uwp build gstreamer-1.0 successfully. Next up: all the other gstreamer recipes in later MRs.