android: Use make-standalone-toolchain.sh from NDK
android: Use make-standalone-toolchain.sh from NDK

This helps avoid having to hardcode internal NDK paths in our build
scripts, making them less fragile.  It also lets us use a combined
sysroot for all dependencies, as the local copy of the toolchain is
guaranteed to be writable.  Net result is that packages can generally
just invoke plain $(CC) without --sysroot, -I, or -L flags.

For background info, see "the easy way" in
$(NDK)/docs/STANDALONE-TOOLCHAIN.html

Also, after openconnect is built, the user is left with a new toolchain
containing many helpful libraries.  It can be used to cross-compile other
programs just by setting $PATH and --host= with appropriate values.

The change was successfully compile-tested with the following NDK releases:
8c, 8d, 8e, 9, 9b, 9c.  For each release, ARM/x86/MIPS targets were built
on Linux using both x86 and x86_64 host binaries.  OSX was not tested.

NDK releases <= 8b were also tested, but they showed a large number of
unrelated build failures due to factors like:

 - Shipping a version of binutils that doesn't recognize the "rrx(s)"
instruction in gmp

 - Calling the x86 binary "i686-android-linux-gcc" instead of
"i686-linux-android-gcc"

 - Using binutils (bfd) ld instead of gold for x86, so the final link
hits the same zlib dependency bug that we see on MIPS

 - Lack of support for the android-14 (4.0/ICS) ABI

Signed-off-by: Kevin Cernekee <3105dbb3dd63fbb4f6371cfb04adc6726f76c56b@gmail.com>
Signed-off-by: David Woodhouse <b460d66aaf00c296a3db1c1d9eeafc081d5f7d70@intel.com>

neshema/openconnect,chadcatlett/openconnect,atyndall/openconnect,nmav/openconnect-mine,liiir1985/openconnect,cernekee/openconnect,liiir1985/openconnect,nmav/openconnect-mine,mveplus/openconnect-client,cernekee/openconnect,brandt/openconnect,liiir1985/openconnect,mveplus/openconnect-client,don-johnny/openconnect,lins05/openconnect,don-johnny/openconnect,brandt/openconnect,mtmiller/openconnect,mtmiller/openconnect,brandt/openconnect,lins05/openconnect,chadcatlett/openconnect,cernekee/openconnect,neshema/openconnect,matlockx/openconnect,cernekee/openconnect,don-johnny/openconnect,nmav/openconnect-mine,mveplus/openconnect-client,mtmiller/openconnect,matlockx/openconnect,chadcatlett/openconnect,atyndall/openconnect,mveplus/openconnect-client,matlockx/openconnect,don-johnny/openconnect,lins05/openconnect,chadcatlett/openconnect,matlockx/openconnect,brandt/openconnect,liiir1985/openconnect,lins05/openconnect,atyndall/openconnect,nmav/openconnect-mine,atyndall/openconnect,neshema/openconnect,neshema/openconnect,mtmiller/openconnect