Enable basic building with ninja (cmake -GNinja)
Enable basic building with ninja (cmake -GNinja)

The `add_custom_target(combined` which was being used to create
`libnupic_core.a` didn't give CMake any information about what was being
output by that operation. Ninja rightfully complained, citing that

    ninja: error: 'src/libnupic_core.a', needed by 'src/_algorithms.so',
missing and no known rule to make it

The solution is to set a variable `LIB_STATIC_NUPICCORE_COMBINED` and
use `add_custom_command` to produce that file instead of
`add_custom_target`.

(Note:
Since `add_custom_command` is one character longer than
`add_custom_target` a large number of lines had to be moved over.
The only substantial change in that part of the diff is using
`add_custom_command` instead of `add_custom_target`, and introducing a
variable `${LIB_STATIC_NUPICCORE_COMBINED}`
)

numenta/nupic.core,neuroidss/nupic.core,subutai/nupic.core,neuroidss/nupic.core,sambitgaan/nupic.core,subutai/nupic.core,scottpurdy/nupic.core,utensil/nupic.core,neuroidss/nupic.core,scottpurdy/nupic.core,neuroidss/nupic.core,breznak/nupic.core,EricSB/nupic.core,lscheinkman/nupic.core,pettitda/nupic.core,pettitda/nupic.core,numenta/htmresearch-core,numenta/htmresearch-core,scottpurdy/nupic.core,ywcui1990/nupic.core,subutai/nupic.core,utensil/nupic.core,EricSB/nupic.core,rhyolight/nupic.core,sambitgaan/nupic.core,rcrowder/nupic.core,EricSB/nupic.core,rhyolight/nupic.core,utensil/nupic.core,utensil/nupic.core,numenta/nupic.core,subutai/nupic.core,numenta/nupic.core,numenta/nupic.core,ywcui1990/nupic.core,breznak/nupic.core,numenta/htmresearch-core,rcrowder/nupic.core,ywcui1990/nupic.core,rcrowder/nupic.core,lscheinkman/nupic.core,pettitda/nupic.core,lscheinkman/nupic.core,rhyolight/nupic.core,pettitda/nupic.core,numenta/htmresearch-core,breznak/nupic.core,EricSB/nupic.core,sambitgaan/nupic.core,rhyolight/nupic.core,sambitgaan/nupic.core,scottpurdy/nupic.core,lscheinkman/nupic.core,rcrowder/nupic.core,breznak/nupic.core,ywcui1990/nupic.core