Fix makefiles to build shared library tests on Windows.

Abstracted away some POSIX-isms that caused MAKE to issue invalid
commands on Windows.  Added a new force-include for the test
programs so that we can use platform-specific macros.

TestSharedLib now builds and cleans up on Windows, though the test
still fails some of the expectations.

Differential Revision: http://reviews.llvm.org/D8277
Patch by: Adrian McCarthy
Issue Tracker: http://llvm.org/pr21727

llvm-svn: 232220