Patching pybind11 to fix premature deletion of python objects

Patching pybind11 to fix premature deletion of python objects
created and wrapped in c++. This patch is based off an open
PR for pybind11.

pybind/pybind11#2839

CMake/External_pybind11.cmake

@@ -6,12 +6,23 @@ if (PYTHON_EXECUTABLE)
   set(pybind_PYTHON_ARGS -DPYTHON_EXECUTABLE:FILEPATH=${PYTHON_EXECUTABLE})
 endif()
 
+# If a patch file exists, apply it
+set (pybind11_patch ${fletch_SOURCE_DIR}/Patches/pybind11)
+if (EXISTS ${pybind11_patch})
+  set(pybind11_PATCH_COMMAND ${CMAKE_COMMAND}
+      -Dpybind11_patch:PATH=${pybind11_patch}
+      -Dpybind11_source:PATH=${fletch_BUILD_PREFIX}/src/pybind11
+      -P ${pybind11_patch}/Patch.cmake
+    )
+endif()
+
 ExternalProject_Add(pybind11
   URL ${pybind11_url}
   URL_MD5 ${pybind11_md5}
   DOWNLOAD_NAME ${pybind11_dlname}
   ${COMMON_EP_ARGS}
   ${COMMON_CMAKE_EP_ARGS}
+  PATCH_COMMAND ${pybind11_PATCH_COMMAND}
   CMAKE_ARGS
     ${COMMON_CMAKE_ARGS}
     -DCMAKE_CXX_STANDARD=17

Patches/pybind11/Patch.cmake

@@ -0,0 +1,31 @@
+#+
+# This file is called as CMake -P script for the patch step of
+# External_pybind11.cmake pybind11_patch and pybind11_source are defined on the command
+# line along with the call.
+#-
+
+# Patching files based off of open PR for pybind11
+# https://github.com/pybind/pybind11/pull/2839
+configure_file(
+  ${pybind11_patch}/cast.h
+  ${pybind11_source}/include/pybind11/
+  COPYONLY
+)
+
+configure_file(
+  ${pybind11_patch}/pybind11.h
+  ${pybind11_source}/include/pybind11/
+  COPYONLY
+)
+
+configure_file(
+  ${pybind11_patch}/detail/class.h
+  ${pybind11_source}/include/pybind11/detail
+  COPYONLY
+)
+
+configure_file(
+  ${pybind11_patch}/detail/common.h
+  ${pybind11_source}/include/pybind11/detail
+  COPYONLY
+)