I'm still investigating what the issue is, but wanted to track this in case users hit this.
```
josh@ip-10-138-85-84:/usr/src/libphutil$ sudo ./scripts/build_xhpast.php
[2015-08-30 22:10:23] EXCEPTION: (CommandException) Command failed with error #2!
COMMAND
'make' -C '/usr/src/libphutil/support/xhpast' 'clean' 'all' 'install'
STDOUT
make: Entering directory `/usr/src/libphutil/support/xhpast'
rm --force xhpast parser.yacc.output libxhpast.a *.o
bison --verbose -Wall --defines=parser.yacc.hpp --output=parser.yacc.cpp parser.y
make: Leaving directory `/usr/src/libphutil/support/xhpast'
STDERR
make: bison: Command not found
make: *** [parser.yacc.hpp] Error 127
at [<phutil>/src/future/exec/ExecFuture.php:416]
phutil(head=master, ref.master=dc0626970a6d)
#0 ExecFuture::resolvex() called at [<phutil>/src/future/exec/execx.php:17]
#1 execx(string, string, string, array) called at [<phutil>/src/parser/xhpast/bin/PhutilXHPASTBinary.php:43]
#2 PhutilXHPASTBinary::build() called at [<phutil>/scripts/build_xhpast.php:6]
```
This is likely caused by D13978.