Create a new libfishcommon convenience static library to build just once
(instead of 12 times!) the majority of the guestfish sources used in the
rest of the C tools (mostly for command line stuff, inspection, and
mount).
The notable exceptions not using libfishcommon are guestfish itself, and
virt-rescue: both need to build at least one of the common sources using
additional CPPFLAGS.
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