Rework compilation of binaries; prepare for automatic dependencies

Rework compilation of binaries; prepare for automatic dependencies

Don't involve user-adjustable variables like CFLAGS until the final
compilation steps, in part so it's clearer where they fit in.  Handle
all of bup's flags independently, and split them into separate
library (helpers) and executable (bin/bup, etc.)  groups via
bup_shared_cflags, helpers_cflags, embed_cflags, etc.

Change the build rules to specify -fPIE for the executables, and
include all dependencies ($^) and use OUTPUT_OPTION in preparation for
automatic C dependency generation.

Add -Wno-unused-command-line-argument for clang.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>