Device/SocketPort: close the socket on error

Previously, the FileEventHandler was unregistered, but the socket was
still there, and Port::GetState() returned READY.  For that reason,
DeviceDescriptor::AutoReopen() would never notice that the connection
was gone.
NEWS: add missing ticket number
Asset: improve HasCursorKeys() on Android

Add the global variable "has_cursor_keys" which is false by default,
but is set to true as soon as XCSoar sees the first cursor key event.
This enables highlighting of focused dialog controls (TRAC #3133).
doc: mention crosstool-ng
OS/Process: preserve environment variables

PATH and LD_LIBRARY_PATH are useful variables that should be passed to
new child processes.
Makefile: re-enable dynamic linking on the Kobo (#3078)

This is a rehash of commit 3c7cb0b51e7c097c5bd0809123a4509cb756f5c0,
which did not work on old Kobo firmware due to an incompatible glibc
version.

This time, we're trying to do better by including our system libraries
(glibc, libstdc++) in the KoboRoot.tgz.  This should work on all
firmware versions and allows using NSS modules, which (hopefully)
finally fixes TRAC #3078.
Kobo: use --rpath instead of LD_LIBRARY_PATH

Choose the library directory at link time, not at runtime.

This avoids yet another glibc problem on the Kobo: due to
LD_LIBRARY_PATH being set, Kobo binaries such as insmod, ifconfig and
wpa_supplicant crashed, because they used the old ld.so with the new
glibc.

With this approach, each binary uses the ld.so/glibc it wants: Kobo
binaries use the Kobo version, and XCSoar uses our own version.
doc/manual: new Kobo toolchain/libs packages
Look/TaskLook: Fix hbGray initialization
doc/manual: document building KoboRoot.tgz
Makefile: work around __float128 problem with clang and GNU libstdc++
Makefile: adjust pkg-config for Raspberry Pi cross-build

Override PKG_CONFIG_LIBDIR and ${prefix}.
Makefile: add variable TARGET_IS_ANDROID
Makefile: remove unused HOSTCC/CXX assignment

This was done before host.mk, and thus had no effect.  Using clang for
host binaries may not be possible sometimes, so let's disable it for
now.
Makefile: add TARGET_CXXFLAGS
Makefile: move flags to LIBCXX_CXXFLAGS_INTERNAL

Don't export these flags to the rest of XCSoar.
Makefile: use LIBSTDCXX_LDFLAGS
Makefile: implement flag LIBCXX=y

Adds clang flag -stdlib=libc++.
test/{ArcApprox,RunFlightListRenderer}: add missing <vector> includes
Math/FastMath: remove deprecated "register" keyword