ENABLE_ALL is not reset in some cases

[linus-sherrill]

1. Start with a clean build area. Run ccmake and select fletch_BUILD_CXX11=OFF and fletch_ENABLE_ALL_PACKAGES= ON.
2. configure and get error

   CMake Error at CMake/External_pybind11.cmake:2 (message):
      CXX11 must be enabled to use pybind11
    Call Stack (most recent call first):
      CMakeLists.txt:285 (include)```
   

3. select fletch_ENABLE_pybind11=OFF and configure. Get same error as in step (2). Stuck in loop of disable option, get error. The option fletch_ENABLE_ALL_PACKAGES is not reset in this case. If manually reset, then it proceeds normally.

If you

1. select fletch_ENABLE_ALL_PACKAGES= on
2. configure
3. select fletch_BUILD_CXX11=off
4. configure - get error as above
5. select fletch_ENABLE_pybind11=OFF
6. configure - no error