MKDOCS: Use specified python executable to run mkdocs (#923)

This fixes an issue with macOS Catalina but probably breaks all the other builds

Signed-off-by: Michael Jackson <mike.jackson@bluequartz.net>

Documentation/ReferenceManual/CMakeLists.txt

@@ -162,7 +162,8 @@ if(SIMPL_USE_MKDOCS)
 
   if(APPLE)
     set(MKDOCS_EXECUTABLE_SCRIPT "${DREAM3DDocRoot}/ReferenceManual/mkdocs")
-    file(WRITE "${MKDOCS_EXECUTABLE_SCRIPT}" "#!/usr/bin/python\n")
+    #file(WRITE "${MKDOCS_EXECUTABLE_SCRIPT}" "#!/usr/bin/python\n")
+    file(WRITE "${MKDOCS_EXECUTABLE_SCRIPT}" "#!${PYTHON_EXECUTABLE}\n")
     file(APPEND "${MKDOCS_EXECUTABLE_SCRIPT}" "# -*- coding: utf-8 -*-\n")
     file(APPEND "${MKDOCS_EXECUTABLE_SCRIPT}" "#    This script is generated automatically from CMake for macOS systems to get the encoding correct.\n")
     file(APPEND "${MKDOCS_EXECUTABLE_SCRIPT}" "#    Generated in code from ${CMAKE_CURRENT_LIST_FILE}\n")