WIP: Convert to use ExternalProject

[mathomp4]

This is a draft PR to capture the experiments with ExternalProject_Add in GFE by myself and @tclune.

At the moment, this seems to build GFE just fine. The issue at the moment is we aren't sure how to do the tests bit.

For example, in current GFE you do:

1. mkdir build && cd build
2. cmake .. -DCMAKE_INSTALL_PREFIX=../install -DCMAKE_PREFIX_PATH=../install -DSKIP_OPENMP=ON
3. make install
4. cmake .. -DCMAKE_INSTALL_PREFIX=../install -DCMAKE_PREFIX_PATH=../install -DSKIP_OPENMP=ON
5. make tests

The second cmake is that the first time around, there was no pFUnit, so no tests were created to build by CMake. Now it sees the installed pFUnit and can make the tests.

Now, with the ExternalProject_Add version you have to do:

1. mkdir build && cd build
2. cmake .. -DCMAKE_INSTALL_PREFIX=../install -DCMAKE_PREFIX_PATH=../install -DSKIP_OPENMP=ON
3. make (note NO install)
4. rm -rf * (the build directory must be removed)
5. cmake .. -DCMAKE_INSTALL_PREFIX=../install -DCMAKE_PREFIX_PATH=../install -DSKIP_OPENMP=ON
6. make gFTL-tests
7. make yaFyaml-tests
8. ...

So you have to run each set of tests individually.

The questions going forward:

1. Can you get ExternalProject_Add to somehow "re-run" CMake for you?
2. Can we make a new custom tests target that when we do make tests will run the tests for each subproject?