toolchain-external: instrument wrapper to warn about unsafe paths

The CodeSourcery toolchains have a very interesting feature: they warn
the user when an unsafe header or library path is used, i.e a path
that will lead host headers or libraries to leak into the build.

This commit adds a similar functionality into our external toolchain
wrapper, so that it can be used with all external toolchains, and can
also be tuned as needed. By default, the external toolchain wrapper
now gives warnings such as:

  arm-linux-gcc: WARNING: unsafe header/library path used in cross-compilation: '-I /usr/foo'
  arm-linux-gcc: WARNING: unsafe header/library path used in cross-compilation: '-L /usr/bleh'

but the compilation continues successfully. One can then easily grep
in his build log to search for occurences of this message.

Optionally, if BR_COMPILER_PARANOID_UNSAFE_PATH is defined in the
environment to a non empty value, the external wrapper will instead
error out and abort the compilation.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Romain Naour <romain.naour@openwide.fr>
Tested-by: Romain Naour <romain.naour@openwide.fr>