ISPC: Support ISPC header generation byproducts and parallel builds

Auxiliary/vim/syntax/cmake.vim

Help/manual/cmake-properties.7.rst

Help/manual/cmake-variables.7.rst

Help/prop_tgt/ISPC_HEADER_DIRECTORY.rst

Help/variable/CMAKE_ISPC_HEADER_DIRECTORY.rst

Modules/CMakeISPCInformation.cmake

Source/cmGeneratorTarget.cxx

Source/cmGeneratorTarget.h

Source/cmGlobalGenerator.cxx

Source/cmGlobalNinjaGenerator.cxx

Source/cmGlobalNinjaGenerator.h

Source/cmLocalGenerator.cxx

Source/cmLocalGenerator.h

Source/cmMakefileTargetGenerator.cxx

Source/cmNinjaTargetGenerator.cxx

Source/cmRulePlaceholderExpander.cxx

Source/cmRulePlaceholderExpander.h

Source/cmTarget.cxx

Tests/ISPC/CMakeLists.txt

Tests/ISPC/Defines/CMakeLists.txt