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.
sigrok-cross-android: Set PKG_CONFIG_LIBDIR globally

PKG_CONFIG_LIBDIR overrides the global directory for .pc files.
This way, no globally installed package files for the build system
will be picked up.  Also, PKG_CONFIG_PATH is unset to prevent any
local build system packages from being found.  Only the packages built
by the script itself should be found.
sigrok-cross-android: Build libusb-1.0