Rename varargs builtins from stdarg.h

This is to fix issue #54.

The __builtin_va_{start,end,copy} are renamed at transformation time
by modifying the associated FunctionDecl.  The case of va_arg(),
however, requires post-translation fixup because these calls use
a distinct subclass of Expr, and there is no way I could find to
override StmtPrinter::VisitVAArgExpr()
Rename __builtin_offsetof() from stddef.h

This is to fix issue #49

The rename of __builtin_offsetof() back to offsetof() is now done via
post-translation #define because these calls use a distinct subclass
of Expr, and there is no way I could find to override
StmtPrinter::VisitOffsetOfExpr().
Refine code as requested by Gary

This commit adds logic to detect whether or not a given transaltion
unit contains any instances of the Expr nodes for __builtin_va_arg or
__builtin_offsetof.  Only if these were seen are the matching defines
added to the boilerplate at the top of each .trans.c.
Merge pull request #65 from PHHargrove/rename-builtins-2

Fix issues #49 and #54