add_module: allow include as SYSTEM (#1416)

pybind11 headers passed via the `pybind11_add_module` CMake
function can now be included as `SYSTEM` includes (`-isystem`).

This allows to set stricter (or experimental) warnings in
calling projects that might throw otherwise in headers
a user of pybind11 can not influence.

docs/compiling.rst

@@ -59,7 +59,7 @@ function with the following signature:
 .. code-block:: cmake
 
     pybind11_add_module(<name> [MODULE | SHARED] [EXCLUDE_FROM_ALL]
-                        [NO_EXTRAS] [THIN_LTO] source1 [source2 ...])
+                        [NO_EXTRAS] [SYSTEM] [THIN_LTO] source1 [source2 ...])
 
 This function behaves very much like CMake's builtin ``add_library`` (in fact,
 it's a wrapper function around that command). It will add a library target
@@ -86,6 +86,10 @@ latter optimizations are never applied in ``Debug`` mode.  If ``NO_EXTRAS`` is
 given, they will always be disabled, even in ``Release`` mode. However, this
 will result in code bloat and is generally not recommended.
 
+By default, pybind11 and Python headers will be included with ``-I``. In order
+to include pybind11 as system library, e.g. to avoid warnings in downstream
+code with warn-levels outside of pybind11's scope, set the option ``SYSTEM``.
+
 As stated above, LTO is enabled by default. Some newer compilers also support
 different flavors of LTO such as `ThinLTO`_. Setting ``THIN_LTO`` will cause
 the function to prefer this flavor if available. The function falls back to

tools/pybind11Tools.cmake

@@ -110,10 +110,10 @@ endfunction()
 
 # Build a Python extension module:
 # pybind11_add_module(<name> [MODULE | SHARED] [EXCLUDE_FROM_ALL]
-#                     [NO_EXTRAS] [THIN_LTO] source1 [source2 ...])
+#                     [NO_EXTRAS] [SYSTEM] [THIN_LTO] source1 [source2 ...])
 #
 function(pybind11_add_module target_name)
-  set(options MODULE SHARED EXCLUDE_FROM_ALL NO_EXTRAS THIN_LTO)
+  set(options MODULE SHARED EXCLUDE_FROM_ALL NO_EXTRAS SYSTEM THIN_LTO)
   cmake_parse_arguments(ARG "${options}" "" "" ${ARGN})
 
   if(ARG_MODULE AND ARG_SHARED)
@@ -130,7 +130,11 @@ function(pybind11_add_module target_name)
 
   add_library(${target_name} ${lib_type} ${exclude_from_all} ${ARG_UNPARSED_ARGUMENTS})
 
-  target_include_directories(${target_name}
+  if(ARG_SYSTEM)
+    set(inc_isystem SYSTEM)
+  endif()
+
+  target_include_directories(${target_name} ${inc_isystem}
     PRIVATE ${PYBIND11_INCLUDE_DIR}  # from project CMakeLists.txt
     PRIVATE ${pybind11_INCLUDE_DIR}  # from pybind11Config
     PRIVATE ${PYTHON_INCLUDE_DIRS})