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Allow for separate installation of tools and target-tools, to aid in packaging.

By default everything the tools are installed into /usr/local/bin but this can
be overridden using PREFIX= or BINDIR= on the make invocation.

To enable this it was necessary to split fex2bin and bin2fex out from $(TOOLS)
into $(FEXC_LINKS), because install(1) does not seem to have a mode which
preserves symlinks so it needs to be done separately.

Supports DESTDIR to allow for convenient installation into a staging dir for
distro packaging convenience.
Signed-off-by: Ian Campbell <ijc@hellion.org.uk>
Acked-by: Peter Korsgaard <peter@korsgaard.com>