Robert Maynard authored Dec 10, 2020 and Brad King committed Dec 14, 2020

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.