mingw README: Mention libusb1 MXE package.
sigrok-cross-mingw: Workaround for MXE/mingw Pulseview link issue.
sigrok-cross-mingw: Installers (NSIS) for sigrok-cli & PulseView.
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
sigrok-cross-android: Bump gettext/pcre/libffi/libzip versions.
sigrok-cross-android: libsigrok: Drop LDFLAGS/CPPFLAGS.

These are not needed, libsigrok's configure script finds everything
correctly via pkg-config now.
sigrok-cross-android: libsigrok: Drop --disable-alsa.

This is no longer needed, libsigrok's configure script correctly detects
that there's no alsa using pkg-config.