Make static linking of xhpast nondefault

Summary:
-static breaks builds on OSX:

ar -crs libxhpast.a astnode.o scanner.lex.o parser.yacc.o
/usr/bin/ranlib: file: libxhpast.a(astnode.o) has no symbols
g++ -fPIC -g -Wall -O3 -minline-all-stringops -static -o xhpast xhpast.cpp libxhpast.a
ld: library not found for -lcrt0.o
collect2: ld returned 1 exit status
make: *** [xhpast] Error 1

Since I think we only need static builds on FB/HPHP machines and we raise better errors now anyway, default to dynamic and provide a static option.

Test Plan: Ran build on OSX.

Reviewers: edward, hsb, btrahan

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T975

Differential Revision: https://secure.phabricator.com/D1858