Running zpaq on an older but not ancient 64-bit Intel server aborts
with an ‘Illegal instruction’ error. Turns out the build expression
was using -march=native to generate distibution binaries...
Change this to more conservative, portable settings which should
cover ‘all’ CPUs. It may run slightly slower — but that at least
implies running.
As a nice side effect, all common compile flags are now back in
`compileFlags` whence they came, and actually used consistently.
