a new version of Building-S2E-on-a-Mac tips, which does not need to patch the codes alone any longer.
Fix the duplicate definition of "uint16".  The patch is derived from upstream
QEMU.
Fix the problem that the weak symbol "s2e_bdrv_fail" cannot be found when
linking and kill some warnings caused by symbols' visibility.

It seems that Apple's support of weak symbol (Apple actually prefers
"__atrribute__ ((weak_import))" to "__attribute__ ((weak))" ) only works well
with dynamic libraries.

Specifying "-undefined dynamic_lookup" can make the static linking work, but it
is said that this is not a hot idea, referring to
http://stackoverflow.com/questions/274753/how-to-make-weak-linking-work-with-gcc