In order to link against an old version of the glibc, we have to pass a
set of flags, some at compile time (CFLAGS), some at link-time
(LDFLAGS).

At compile-time, we set the sysroot to root/link-only, in order to have
the compiler look there first for header.
At link-time, we manually specify `-Lroot/link-only/lib` and reset
sysroot to /.
However, this often creates issues at configure-time, since in this
situation CFLAGS and LDFLAGS are often use at the same time, leading to
usage of / as sysroot.

This commit fixes the problem by ensuring that the `--sysroot` flag in
LDFLAGS will be available only to the link, by simply adding `-Wl,`.

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In ui/qt, we patch `linux-clang/qmake.conf` in order to build `qmake`
using the appropriate flags. However, these changes used to get
installed, leading to hardcoding paths from the CI.

This commit removes the offending lines after the install phase.

Sadly, QtCreator plugins need includes from QtCreator source code.  In
future, we'll need to be able to express dependencies on the source code
of other components. For now, just invoke `orc clone ui/qt-creator`.

Introduce host-c-toolchain, host-cxx-toolchain, and host-libcxx
metacomponents, meant to be used to depend on a generic c/c++/libc++ is
available.
Also introduce gcc-host-toolchain, providing host gcc and binutils.

This variable is needed to make toolchain/host/gcc work with e.g.
Ubuntu and other distros.
Without it crt{1,i,n}.o, Scrt.o and libc.so cannot be found.

GCC 4.9.3 seems to leak `CFLAGS` for the compiler itself into
`CFLAGS_FOR_TARGET`, leading to trying to link `libgcc` (and others)
against legacy-glibc.