android: Remove dependency on files outside the openconnect repo

The Android build tries to run "git clone --reference ../../../gnutls".
This only works if the user happens to have another copy of the gnutls
repo in the right place.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
android: Hack around GnuTLS build breakage

GnuTLS wants to build e.g. src/tests.c and src/common.c into
gnutls-cli-debug, but these files do not compile unless DHE and ECDHE
are enabled.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
android: Override vpnc-script check in "configure"

If we're cross compiling for some other target, we don't particularly care
whether the build host has a valid vpnc-script installed locally.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
android: Add the correct toolchain directory to $PATH

On the 64-bit NDK, the toolchain binaries live under:

toolchains/arm-linux-androideabi-4.7/prebuilt/linux-x86_64/bin

So hardcoding prebuilt/linux-x86/bin would cause build failures.

This probably affects the OSX NDK as well, but I didn't check whether my
fix works for that case.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
android: Run autogen.sh if configure script is missing

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>