Configure ELAST for MinGW

Summary:
Use _LIBCPP_MSVCRT_LIKE while configuring ELAST, so MinGW gets the same
configuration as MSVC.

Reviewers: compnerd, srhines, danalbert, mstorsjo

Subscribers: christof, ldionne, cfe-commits

Differential Revision: https://reviews.llvm.org/D48731

llvm-svn: 335916