do not attempt to download & install tkdnd or Tcl/Tk

diff --git cmake/external_projects/tcltk.cmake cmake/external_projects/tcltk.cmake
index 637c1899..1dd10dd2 100644
--- cmake/external_projects/tcltk.cmake
+++ cmake/external_projects/tcltk.cmake
@@ -64,116 +64,8 @@
 
 else(SKBUILD)
 if(APPLE)
-  set(tcl_prefix ${CMAKE_INSTALL_PREFIX})
-  # URL "http://sourceforge.net/projects/tcl/files/Tcl/8.6.9/tcl8.6.9-src.tar.gz"
-  # URL_MD5 aa0a121d95a0e7b73a036f26028538d4
-  ExternalProject_Add(project_tcl
-    URL "https://github.com/NGSolve/tcl/archive/7769161.zip"
-    URL_MD5 1131f188dd26944df557913c475d43b4
-    DOWNLOAD_DIR ${CMAKE_CURRENT_SOURCE_DIR}/external_dependencies
-    UPDATE_COMMAND ""
-    CONFIGURE_COMMAND ../project_tcl/macosx/configure --enable-threads --enable-framework --prefix=${tcl_prefix} --libdir=${tcl_prefix}/Contents/Frameworks --bindir=${tcl_prefix}/Contents/Frameworks/Tcl.framework/bin
-    BUILD_COMMAND make -j4 binaries libraries
-    INSTALL_COMMAND make install-binaries install-headers install-libraries install-private-headers
-    ${SUBPROJECT_ARGS}
-    )
-
-  # URL "http://sourceforge.net/projects/tcl/files/Tcl/8.6.9/tk8.6.9.1-src.tar.gz"
-  # URL_MD5 9efe3976468352dc894dae0c4e785a8e
-  ExternalProject_Add(project_tk
-    DEPENDS project_tcl
-    URL "https://github.com/NGSolve/tk/archive/e7c2bc7.zip"
-    URL_MD5 94044140d4826069c22f1c60cedb6e59
-    DOWNLOAD_DIR ${CMAKE_CURRENT_SOURCE_DIR}/external_dependencies
-    UPDATE_COMMAND ""
-    CONFIGURE_COMMAND ../project_tk/macosx/configure --enable-aqua=yes --enable-threads --enable-framework --prefix=${tcl_prefix} --libdir=${tcl_prefix}/Contents/Frameworks --bindir=${tcl_prefix}/Contents/Frameworks/Tcl.framework/bin --with-tcl=${tcl_prefix}/Contents/Frameworks/Tcl.framework
-    BUILD_COMMAND make -j4 binaries libraries
-    INSTALL_COMMAND make install-binaries install-headers install-libraries install-private-headers
-    ${SUBPROJECT_ARGS}
-    )
-
-  ExternalProject_Add(project_tkdnd
-    URL "https://src.fedoraproject.org/repo/pkgs/tkdnd/tkdnd2.8-src.tar.gz/a6d47a996ea957416469b12965d4db91/tkdnd2.8-src.tar.gz"
-    URL_MD5 a6d47a996ea957416469b12965d4db91
-    DEPENDS project_tcl project_tk
-    DOWNLOAD_DIR ${CMAKE_CURRENT_SOURCE_DIR}/external_dependencies
-    PATCH_COMMAND  patch < ${CMAKE_CURRENT_LIST_DIR}/tkdnd_macosx.patch
-    UPDATE_COMMAND "" # Disable update
-    BUILD_IN_SOURCE 1
-    CMAKE_ARGS
-           -DCMAKE_INSTALL_PREFIX=${CMAKE_INSTALL_PREFIX}/Contents/MacOS
-	   -DTCL_INCLUDE_PATH=${CMAKE_INSTALL_PREFIX}/Contents/Frameworks/Tcl.framework/Headers
-	   -DTK_INCLUDE_PATH=${CMAKE_INSTALL_PREFIX}/Contents/Frameworks/Tk.framework/Headers
-    ${SUBPROJECT_ARGS}
-  )
 
-  list(APPEND NETGEN_DEPENDENCIES project_tcl project_tk project_tkdnd)
-  list(APPEND CMAKE_PREFIX_PATH ${CMAKE_INSTALL_PREFIX}/Contents/Frameworks)
-  set(TCL_INCLUDE_PATH ${CMAKE_INSTALL_PREFIX}/Contents/Frameworks/Tcl.framework/Headers)
-  set(TCL_LIBRARY ${CMAKE_INSTALL_PREFIX}/Contents/Frameworks/Tcl.framework)
-  set(TK_LIBRARY ${CMAKE_INSTALL_PREFIX}/Contents/Frameworks/Tk.framework)
-  set(TK_INCLUDE_PATH ${CMAKE_INSTALL_PREFIX}/Contents/Frameworks/Tk.framework/Headers)
-
-  set(TCL_STUB_LIBRARY ${CMAKE_INSTALL_PREFIX}/Contents/Frameworks/Tcl.framework/libtclstub8.6.a)
-  set(TK_STUB_LIBRARY ${CMAKE_INSTALL_PREFIX}/Contents/Frameworks/Tk.framework/libtkstub8.6.a)
-
-#   # use system tcl/tk
-#   if((${PYTHON_VERSION_STRING} VERSION_EQUAL "3.7") OR (${PYTHON_VERSION_STRING} VERSION_GREATER "3.7"))
-#     # fetch tcl/tk sources to match the one used in Python 3.7
-#     ExternalProject_Add(project_tcl
-#       URL "https://prdownloads.sourceforge.net/tcl/tcl8.6.8-src.tar.gz"
-#       URL_MD5 81656d3367af032e0ae6157eff134f89
-#       DOWNLOAD_DIR ${CMAKE_CURRENT_SOURCE_DIR}/external_dependencies
-#       UPDATE_COMMAND "" # Disable update
-#       CONFIGURE_COMMAND ""
-#       BUILD_COMMAND ""
-#       INSTALL_COMMAND ""
-#       )
-#     ExternalProject_Add(project_tk
-#       URL "https://prdownloads.sourceforge.net/tcl/tk8.6.8-src.tar.gz"
-#       URL_MD5 5e0faecba458ee1386078fb228d008ba
-#       DOWNLOAD_DIR ${CMAKE_CURRENT_SOURCE_DIR}/external_dependencies
-#       UPDATE_COMMAND "" # Disable update
-#       CONFIGURE_COMMAND ""
-#       BUILD_COMMAND ""
-#       INSTALL_COMMAND ""
-#       )
-# 
-#     get_filename_component(PYTHON_LIB_DIR ${PYTHON_LIBRARY} DIRECTORY)
-#     find_library(TCL_LIBRARY libtcl8.6.dylib PATHS ${PYTHON_LIB_DIR} NO_DEFAULT_PATH)
-#     find_library(TK_LIBRARY libtk8.6.dylib PATHS ${PYTHON_LIB_DIR} NO_DEFAULT_PATH)
-# 
-#     set(TCL_DIR ${CMAKE_CURRENT_BINARY_DIR}/dependencies/src/project_tcl)
-#     set(TK_DIR ${CMAKE_CURRENT_BINARY_DIR}/dependencies/src/project_tk)
-#     set(TCL_INCLUDE_PATH "${TCL_DIR}/generic;${TCL_DIR}/macosx")
-#     set(TK_INCLUDE_PATH "${TK_DIR}/generic;${TK_DIR}/macosx;${TK_DIR}/xlib")
-#     string(REPLACE ";" "$<SEMICOLON>" TCL_INC "${TCL_INCLUDE_PATH}")
-#     string(REPLACE ";" "$<SEMICOLON>" TK_INC "${TK_INCLUDE_PATH}")
-# 
-#     ExternalProject_Add(project_tkdnd
-#       URL "http://sourceforge.net/projects/tkdnd/files/TkDND/TkDND%202.8/tkdnd2.8-src.tar.gz"
-#       URL_MD5 a6d47a996ea957416469b12965d4db91
-#       DEPENDS project_tcl project_tk
-#       DOWNLOAD_DIR ${CMAKE_CURRENT_SOURCE_DIR}/external_dependencies
-#       PATCH_COMMAND  patch < ${CMAKE_CURRENT_LIST_DIR}/tkdnd_macosx.patch
-#       UPDATE_COMMAND "" # Disable update
-#       BUILD_IN_SOURCE 1
-#       CMAKE_ARGS
-# 	      -DCMAKE_INSTALL_PREFIX=${CMAKE_INSTALL_PREFIX}/Contents/MacOS
-# 	      -DTCL_INCLUDE_PATH=${TCL_INC}
-# 	      -DTK_INCLUDE_PATH=${TK_INC}
-# 	      -DTK_LIBRARY=${TK_LIBRARY}
-# 	      -DTCL_LIBRARY=${TCL_LIBRARY}
-#       LOG_DOWNLOAD 1
-#       LOG_CONFIGURE 1
-#       LOG_BUILD 1
-#       LOG_INSTALL 1
-#     )
-#       
-# list(APPEND NETGEN_DEPENDENCIES project_tkdnd)
-#   else()
-#     find_package(TCL 8.5 REQUIRED)
-#   endif()
+find_package(TCL 8.5 REQUIRED)
 
 elseif(WIN32)