Arch-indep CMake packaging (#764)

* Arch-indep CMake packaging

Since pybind11 is a header-only library, the CMake packaging does not have to carry any architecture specific checks. Without this patch, the detection of pybind11 will fail on 32-bit architectures if the project was built on a 64-bit machine and vice-versa. This fix is similar to what is applied to `Eigen` and other header-only C++ libraries.

CMakeLists.txt

@@ -105,9 +105,14 @@ if (PYBIND11_INSTALL)
   configure_package_config_file(tools/${PROJECT_NAME}Config.cmake.in
                                 "${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}Config.cmake"
                                 INSTALL_DESTINATION ${PYBIND11_CMAKECONFIG_INSTALL_DIR})
+  # Remove CMAKE_SIZEOF_VOID_P from ConfigVersion.cmake since the library does
+  # not depend on architecture specific settings or libraries.
+  set(_PYBIND11_CMAKE_SIZEOF_VOID_P ${CMAKE_SIZEOF_VOID_P})
+  unset(CMAKE_SIZEOF_VOID_P)
   write_basic_package_version_file(${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}ConfigVersion.cmake
                                    VERSION ${${PROJECT_NAME}_VERSION}
                                    COMPATIBILITY AnyNewerVersion)
+  set(CMAKE_SIZEOF_VOID_P ${_PYBIND11_CMAKE_SIZEOF_VOID_P})
   install(FILES ${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}Config.cmake
                 ${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}ConfigVersion.cmake
                 tools/FindPythonLibsNew.cmake