CMake SYSTEM INTERFACE

[severinstrobl]

When adding pybind11 to a CMake project via add_subdirectory(), the SYSTEM keyword is added to the specification of the include directory in

pybind11/CMakeLists.txt

Lines 185 to 186 in fac23b6

	target_include_directories(
	pybind11_headers ${pybind11_system} INTERFACE $<BUILD_INTERFACE:${pybind11_INCLUDE_DIR}>

While this is certainly the right way to handle include directories in sub-projects, it can result in problems for some environments: For HPC systems, it is not uncommon for module files to add their respective include path to the CPATH environment variable. When building a CMake project shipping pybind11 as a third-party module and adding it via add_subdirectory(), a different installation of pybind11 may be used during compilation in case an external module providing pybind11 is loaded at the same time. This is a result of the default ordering of include paths by the compiler, where entries in $CPATH are searched before the ones provided via -isystem (resulting from the SYSTEM INTERFACE).
Admittedly, this is a rather obscure problem, but maybe a workaround exists for such setups. If not, would it be possible to allow disabling the addition of SYSTEM in pybind11 via an configuration option? As a interim fix, in my version of pybind11 I added such an option to selectively control pybind11_system

pybind11/CMakeLists.txt

Line 87 in fac23b6

set(pybind11_system SYSTEM)

via

  if(NOT DEFINED pybind11_SYSTEM_INTERFACE OR ${pybind11_SYSTEM_INTERFACE})
    set(pybind11_system SYSTEM)
  else()
    set(pybind11_system "")
  endif()

allowing me to disable SYSTEM via set(pybind11_SYSTEM_INTERFACE off).

I'd be very happy to hear any suggestions or comments regarding this issue!