MAINT: better warning message when running build_src from sdist
MAINT: better warning message when running build_src from sdist

Addresses comment of @stefanv on gh-7131.

MSeifert04/numpy,pizzathief/numpy,abalkin/numpy,pizzathief/numpy,pizzathief/numpy,ahaldane/numpy,jorisvandenbossche/numpy,ahaldane/numpy,chatcannon/numpy,anntzer/numpy,anntzer/numpy,solarjoe/numpy,rgommers/numpy,shoyer/numpy,behzadnouri/numpy,tynn/numpy,charris/numpy,jakirkham/numpy,simongibbons/numpy,rgommers/numpy,gfyoung/numpy,charris/numpy,ahaldane/numpy,bringingheavendown/numpy,pdebuyl/numpy,seberg/numpy,pizzathief/numpy,charris/numpy,grlee77/numpy,anntzer/numpy,endolith/numpy,b-carter/numpy,numpy/numpy,solarjoe/numpy,simongibbons/numpy,madphysicist/numpy,jakirkham/numpy,MSeifert04/numpy,madphysicist/numpy,jorisvandenbossche/numpy,shoyer/numpy,shoyer/numpy,WarrenWeckesser/numpy,pdebuyl/numpy,mhvk/numpy,pdebuyl/numpy,b-carter/numpy,grlee77/numpy,charris/numpy,numpy/numpy,chatcannon/numpy,mhvk/numpy,pizzathief/numpy,ahaldane/numpy,jakirkham/numpy,anntzer/numpy,pbrod/numpy,pdebuyl/numpy,mattip/numpy,WarrenWeckesser/numpy,seberg/numpy,ssanderson/numpy,bringingheavendown/numpy,endolith/numpy,WarrenWeckesser/numpy,jakirkham/numpy,simongibbons/numpy,simongibbons/numpy,Eric89GXL/numpy,endolith/numpy,seberg/numpy,behzadnouri/numpy,ssanderson/numpy,seberg/numpy,rgommers/numpy,pbrod/numpy,shoyer/numpy,WarrenWeckesser/numpy,madphysicist/numpy,madphysicist/numpy,numpy/numpy,bringingheavendown/numpy,mhvk/numpy,MSeifert04/numpy,mhvk/numpy,tynn/numpy,behzadnouri/numpy,grlee77/numpy,pbrod/numpy,mhvk/numpy,Eric89GXL/numpy,jakirkham/numpy,solarjoe/numpy,ssanderson/numpy,MSeifert04/numpy,jorisvandenbossche/numpy,grlee77/numpy,gfyoung/numpy,numpy/numpy,shoyer/numpy,madphysicist/numpy,rgommers/numpy,abalkin/numpy,mattip/numpy,WarrenWeckesser/numpy,jorisvandenbossche/numpy,pbrod/numpy,tynn/numpy,Eric89GXL/numpy,jorisvandenbossche/numpy,abalkin/numpy,MSeifert04/numpy,grlee77/numpy,mattip/numpy,ahaldane/numpy,Eric89GXL/numpy,b-carter/numpy,chatcannon/numpy,pbrod/numpy,gfyoung/numpy,mattip/numpy,endolith/numpy,simongibbons/numpy