d3dcompiler_qt: Fix export warning on MSVC

MSVC warns about multiple symbols when using extern "C", but MinGW
requires it. #ifdef it for MinGW.

Change-Id: I8b16ab4ea129312787090e042115d8f709fdd814
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>
d3dcompiler_qt: Remove directory creation inside the proxy library

Use of mkpath/mkdir is removed in order to make the proxy less
invasive when there is no compiler service running. Creation of the
directory structure is fully the service's responsibility.
Service availability is now only checked once, at the first invocation of
D3DCompile, as it is expected to be started before the application.

Change-Id: Ib8c4f062c418497c2253daf524654e1db30dae47
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>