sigrok-cross-android: Stop on errors
sigrok-cross-android: Update zlib version
sigrok-cross-android: Ignore build products
sigrok-cross-android: Avoid building C++ libraries

The Android NDK doesn't ship C++ includes, so skip building C++
support in libpcre and libusb (we don't need it anyway).
sigrok-cross-android: Fix iconv build
sigrok-cross-android: Fix gettext build
sigrok-cross-android: Fix glib build
sigrok-cross-android: Build libsigrok
sigrok-cross-android: Silence unzip.
sigrok-cross-android: Add PKG_CONFIG_PATH for glib.

Among other things, this fixes the build wrt ffi.h not being found.

  CC       libgobject_2_0_la-gclosure.lo
gclosure.c:29:17: fatal error: ffi.h: No such file or directory
sigrok-cross-mingw: Drop libusb-1.0 build, now in MXE.

libusb-1.0 is now available from MXE (www.mxe.cc) like all of the other
dependencies, so we don't have to build it manually anymore.