The target property `ISPC_HEADER_SUFFIX` and associated global
variable now can control the suffix used when generating the
C/C++ interoperability ISPC headers.
In addition the default suffix is now "_ispc.h" which matches the
common convention that the ISPC compiler team uses and recommends.
The MSVC linker needs to know what MSVC runtime a shared library
needs. ISPC objects don't have a '/DIRECTIVE' entry for the
MSVC runtime as they have no dependency on it. Therefore
we need to add a C or C++ source to each shared library so
the MSVC linker knows what runtime to embed
