pbc_to_exe: missing runtime dependency

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when running make, pbc_to_exe occasionally fails with the following error:

PackFile_unpack: Buffer length 0 is shorter than PACKFILE_HEADER_BYTES 18.
current instr.: 'handle_args' pc 223 (tools/dev/pbc_to_exe.pir:233)
make: *** [pbc_to_exe] Error 1
make: *** Waiting for unfinished jobs....

i've not been able to reproduce this consistently, but i think it occurs when pmc_to_exe is invoked as part of the build process, and tries to load Getopt/Obj.pbc before it's been fully compiled. the file exists so open() doesn't fail, but is still zero-length.