build folly master on mac

1. build tutorial from github

You may also use folly/build/bootstrap-osx-homebrew.sh
to build against master:
cd folly 
./build/bootstrap-osx-homebrew.sh 
make 
make check

2. but it never worked out

I've met several errors when compile folly.
First, compile complain about not finding preadv and pwritev, here is the trick to bypass this error:
1) edit file config.h.in
2) find lines that #undef HAS_PREADV and #undef HAS_PWRITEV
3) comment both of them and add lines:
#define HAS_PREADV 0
#define HAS_PWRITEV 0
Second, I got compilation error as '#error "SSE4.2 instruction set not enabled"', this problem is solved by add -msse4.2 cppflags as

./configure CPPFLAGS="-msse4.2"

Finally, 'make' seems success, but 'make check' failed.
Anyway, the compilation is done.