Allow external definitions of functions defined in .pxd files.
Allow external definitions of functions defined in .pxd files.

From Jeroen Demeyer.

hickford/cython,encukou/cython,fabianrost84/cython,fabianrost84/cython,marscher/cython,hickford/cython,hhsprings/cython,dahebolangkuan/cython,andreasvc/cython,ChristopherHogan/cython,JelleZijlstra/cython,scoder/cython,ChristopherHogan/cython,marscher/cython,mcanthony/cython,cython/cython,madjar/cython,hickford/cython,encukou/cython,c-blake/cython,andreasvc/cython,fabianrost84/cython,slonik-az/cython,cython/cython,scoder/cython,mcanthony/cython,madjar/cython,roxyboy/cython,JelleZijlstra/cython,slonik-az/cython,marscher/cython,mrGeen/cython,madjar/cython,achernet/cython,marscher/cython,acrispin/cython,acrispin/cython,dahebolangkuan/cython,hickford/cython,JelleZijlstra/cython,fperez/cython,madjar/cython,ChristopherHogan/cython,slonik-az/cython,encukou/cython,slonik-az/cython,fperez/cython,roxyboy/cython,mcanthony/cython,acrispin/cython,slonik-az/cython,achernet/cython,andreasvc/cython,fperez/cython,da-woods/cython,marscher/cython,roxyboy/cython,c-blake/cython,achernet/cython,andreasvc/cython,scoder/cython,hhsprings/cython,scoder/cython,mrGeen/cython,mrGeen/cython,hhsprings/cython,cython/cython,roxyboy/cython,fabianrost84/cython,c-blake/cython,hhsprings/cython,hhsprings/cython,mrGeen/cython,roxyboy/cython,fperez/cython,c-blake/cython,andreasvc/cython,fabianrost84/cython,mcanthony/cython,acrispin/cython,JelleZijlstra/cython,mrGeen/cython,hickford/cython,JelleZijlstra/cython,cython/cython,achernet/cython,da-woods/cython,mcanthony/cython,da-woods/cython,encukou/cython,dahebolangkuan/cython,achernet/cython,dahebolangkuan/cython,madjar/cython,da-woods/cython,fperez/cython,acrispin/cython,dahebolangkuan/cython,c-blake/cython,encukou/cython