[cmake] copy parts of pybind11/CMakeLists.txt into DunePybindxiMacros.cmake

cmake/modules/DunePybindxiMacros.cmake

-# File for module specific CMake tests.
+# The code below is a renamed copy of parts of ../../pybind11/CMakeLists.txt,
+# see ../../pybind11/LICENSE for license information.
+
+
+# Add a CMake parameter for choosing a desired Python version
+set(DUNE_PYBINDXI_PYTHON_VERSION "" CACHE STRING "Python version to use for dune-pybindxi")
+
+set(Python_ADDITIONAL_VERSIONS 3.4 3.5 3.6 3.7)
+include(FindPythonLibsNew)
+find_package(PythonLibsNew ${PYBIND11_PYTHON_VERSION} REQUIRED)
+
+set(PYTHON_INCLUDE_DIRS ${PYTHON_INCLUDE_DIRS} CACHE INTERNAL "")
+set(PYTHON_LIBRARIES ${PYTHON_LIBRARIES} CACHE INTERNAL "")
+set(PYTHON_MODULE_PREFIX ${PYTHON_MODULE_PREFIX} CACHE INTERNAL "")
+set(PYTHON_MODULE_EXTENSION ${PYTHON_MODULE_EXTENSION} CACHE INTERNAL "")
+
+include(CheckCXXCompilerFlag)
+if(NOT MSVC AND NOT DUNE_PYBINDXI_CPP_STANDARD)
+  #check_cxx_compiler_flag("-std=c++14" HAS_CPP14_FLAG)
+  check_cxx_compiler_flag("-std=c++11" HAS_CPP11_FLAG)
+
+  #if (HAS_CPP14_FLAG)
+  #  set(DUNE_PYBINDXI_CPP_STANDARD -std=c++14)
+  #elseif (HAS_CPP11_FLAG)
+  if (HAS_CPP11_FLAG)
+    set(DUNE_PYBINDXI_CPP_STANDARD -std=c++11)
+  else()
+    message(FATAL_ERROR "Unsupported compiler -- pybind11 requires C++11 support!")
+  endif()
+
+  set(DUNE_PYBINDXI_CPP_STANDARD ${DUNE_PYBINDXI_CPP_STANDARD} CACHE STRING
+          "C++ standard flag, e.g. -std=c++11 or -std=c++14. Defaults to C++11." FORCE)
+endif()
+
+include(DunePybindxiUtils)