Document building core lib separately

[molpopgen]

mkdir build
cd build
cmake -DCMAKE_ARCHIVE_OUTPUT_DIRECTORY=`pwd`/../lib ..
cmake --build . --target fwdpy11core

The static archive will then be in the repo_root/lib, making it available to the unit test suite. A bit inconvenient, but it beats compiling the entire Python package when doing C++-level testing.

[molpopgen]

Update given some recent changes. The most expedient method is:

cmake -S. -Bbuild -DCMAKE_BUILD_TYPE=Release
cmake --build build -t fwdpy11core -j X
# Or
make -C build fwdpy11core -j X

Where X is the number of threads to use.