RFR 8217268: Windows tests fail to build

[Maurizio Cimadamore]

Hi,
This is an official RFR for this patch:

http://cr.openjdk.java.net/~mcimadamore/panama/8217268/

This takes care of the warnings causing build failure when running tests 
with make; it also addresses the problem of copying libclang.dll in the 
right folder (I did that in a very minimalistic way, to prevent issues 
in other platforms, we can revise this code at a later point if needed).

With this, Windows builds and all tests pass except for two:

SystemHeadersTest
TestJextractFFI

These tests depend on system headers - and clang by default, on Windows 
resolves system headers using the standard MSVC install path. If MSVC is 
not installed, clang will fail to resolve system headers (unless an 
explicit -I option is passed to jextract). I don't think there's a way 
to fix these tests in a way that they will work in our internal 
build/test setup - given that the machines in such setup do not have a 
standard MSVC install.

That said, I think this fix makes Windows support much more reliable 
when it comes to testing.

Cheers
Maurizio