ostree-prepare-root: Allow building statically with musl

ostree-prepare-root: Allow building statically with musl

If the `--with-static-compiler=musl-gcc` configure flag is given.

ostree-prepare-root can be used as init in a system without a populated
/lib.  To support this use case we need to link statically as we will be
unable to locate libc.so at run time if it's not installed in /lib.

We support building ostree-prepare-root with a different compiler to the
rest of ostree so we can use musl rather than glibc.  This reduces the size
of the executable significantly: from ~700K -> ~30K.  We have to use
`_SCRIPTS` here to get autotools to install this as an executable but
without generating rules to make it itself which we have specified
manually.

See https://lists.gnu.org/archive/html/help-gnu-utils/2007-01/msg00007.html
for advice on using autotools in this manner.

Closes: #477
Approved by: cgwalters