DOC: Specifically mention the C99 requirement in 'Building from source'
DOC: Specifically mention the C99 requirement in 'Building from source'

seberg/numpy,endolith/numpy,mhvk/numpy,mhvk/numpy,numpy/numpy,charris/numpy,anntzer/numpy,anntzer/numpy,jakirkham/numpy,mhvk/numpy,rgommers/numpy,mhvk/numpy,pdebuyl/numpy,numpy/numpy,endolith/numpy,pdebuyl/numpy,anntzer/numpy,pdebuyl/numpy,seberg/numpy,rgommers/numpy,jakirkham/numpy,numpy/numpy,rgommers/numpy,mhvk/numpy,jakirkham/numpy,mattip/numpy,jakirkham/numpy,endolith/numpy,mattip/numpy,rgommers/numpy,seberg/numpy,anntzer/numpy,pdebuyl/numpy,charris/numpy,endolith/numpy,jakirkham/numpy,charris/numpy,charris/numpy,mattip/numpy,mattip/numpy,numpy/numpy,seberg/numpy