auto merge of #17449 : mcoffin/rust/master, r=alexcrichton
auto merge of #17449 : mcoffin/rust/master, r=alexcrichton

Right now, libuv will **always** be built for the host system (at least when building on OSX) because the information about the cross compiler is never actually passed to GYP. I don't know how anybody has been managing to build cross compilers with this.

Note that, at least on OSX, there is a bug in GYP that will send clang flags to non-clang compilers and it will still attempt to use Xcode's libtool, so this doesn't completely fix the problem of cross-compiling on an OSX host, but it's a start.

andars/rust,sae-bom/rust,carols10cents/rust,jroesch/rust,ktossell/rust,ejjeong/rust,omasanori/rust,quornian/rust,sae-bom/rust,seanrivera/rust,jroesch/rust,graydon/rust,ejjeong/rust,richo/rust,omasanori/rust,kwantam/rust,omasanori/rust,aepsil0n/rust,quornian/rust,AerialX/rust,cllns/rust,andars/rust,dwillmer/rust,pshc/rust,ktossell/rust,zaeleus/rust,krzysz00/rust,TheNeikos/rust,zachwick/rust,philyoon/rust,zachwick/rust,omasanori/rust,aepsil0n/rust,dwillmer/rust,victorvde/rust,mdinger/rust,hauleth/rust,nwin/rust,jashank/rust,aepsil0n/rust,ebfull/rust,defuz/rust,l0kod/rust,kwantam/rust,zubron/rust,reem/rust,kwantam/rust,AerialX/rust-rt-minimal,rprichard/rust,miniupnp/rust,ktossell/rust,P1start/rust,aneeshusa/rust,XMPPwocky/rust,mdinger/rust,l0kod/rust,seanrivera/rust,jroesch/rust,victorvde/rust,carols10cents/rust,mihneadb/rust,ebfull/rust,reem/rust,quornian/rust,hauleth/rust,mvdnes/rust,vhbit/rust,aneeshusa/rust,rprichard/rust,andars/rust,richo/rust,ejjeong/rust,bombless/rust-docs-chinese,jroesch/rust,andars/rust,mdinger/rust,AerialX/rust-rt-minimal,hauleth/rust,barosl/rust,rohitjoshi/rust,aneeshusa/rust,graydon/rust,richo/rust,seanrivera/rust,reem/rust,ejjeong/rust,l0kod/rust,ejjeong/rust,AerialX/rust-rt-minimal,pshc/rust,P1start/rust,cllns/rust,pshc/rust,kwantam/rust,gifnksm/rust,0x73/rust,dwillmer/rust,untitaker/rust,TheNeikos/rust,zubron/rust,miniupnp/rust,ruud-v-a/rust,mihneadb/rust,rohitjoshi/rust,graydon/rust,rohitjoshi/rust,gifnksm/rust,robertg/rust,AerialX/rust,mahkoh/rust,defuz/rust,carols10cents/rust,quornian/rust,mihneadb/rust,untitaker/rust,rprichard/rust,victorvde/rust,GBGamer/rust,vhbit/rust,KokaKiwi/rust,zubron/rust,aidancully/rust,zachwick/rust,KokaKiwi/rust,seanrivera/rust,omasanori/rust,barosl/rust,aneeshusa/rust,vhbit/rust,XMPPwocky/rust,mahkoh/rust,GBGamer/rust,philyoon/rust,TheNeikos/rust,rohitjoshi/rust,KokaKiwi/rust,cllns/rust,pelmers/rust,krzysz00/rust,zaeleus/rust,KokaKiwi/rust,zaeleus/rust,omasanori/rust,mvdnes/rust,dwillmer/rust,cllns/rust,miniupnp/rust,nwin/rust,ktossell/rust,l0kod/rust,zubron/rust,graydon/rust,robertg/rust,miniupnp/rust,GBGamer/rust,0x73/rust,pelmers/rust,kwantam/rust,avdi/rust,graydon/rust,0x73/rust,nwin/rust,dinfuehr/rust,sae-bom/rust,krzysz00/rust,mahkoh/rust,robertg/rust,sae-bom/rust,TheNeikos/rust,jroesch/rust,GBGamer/rust,jroesch/rust,philyoon/rust,aidancully/rust,TheNeikos/rust,XMPPwocky/rust,gifnksm/rust,untitaker/rust,XMPPwocky/rust,cllns/rust,zubron/rust,ruud-v-a/rust,jroesch/rust,pshc/rust,jashank/rust,ejjeong/rust,barosl/rust,ebfull/rust,dinfuehr/rust,mihneadb/rust,AerialX/rust,AerialX/rust,P1start/rust,jashank/rust,jashank/rust,bombless/rust,avdi/rust,graydon/rust,P1start/rust,rprichard/rust,vhbit/rust,XMPPwocky/rust,vhbit/rust,pelmers/rust,zubron/rust,andars/rust,untitaker/rust,zachwick/rust,dinfuehr/rust,GBGamer/rust,dwillmer/rust,sae-bom/rust,mvdnes/rust,jroesch/rust,victorvde/rust,barosl/rust,reem/rust,P1start/rust,ebfull/rust,kimroen/rust,avdi/rust,emk/rust,quornian/rust,nwin/rust,carols10cents/rust,0x73/rust,aepsil0n/rust,robertg/rust,GBGamer/rust,kimroen/rust,nwin/rust,bombless/rust,dwillmer/rust,krzysz00/rust,victorvde/rust,nwin/rust,l0kod/rust,avdi/rust,reem/rust,vhbit/rust,dwillmer/rust,miniupnp/rust,aidancully/rust,richo/rust,GBGamer/rust,ktossell/rust,avdi/rust,sae-bom/rust,nwin/rust,gifnksm/rust,untitaker/rust,quornian/rust,aneeshusa/rust,jashank/rust,vhbit/rust,hauleth/rust,seanrivera/rust,barosl/rust,ruud-v-a/rust,pelmers/rust,hauleth/rust,zaeleus/rust,cllns/rust,bombless/rust,pelmers/rust,dinfuehr/rust,ruud-v-a/rust,zaeleus/rust,0x73/rust,dinfuehr/rust,defuz/rust,rohitjoshi/rust,pshc/rust,mihneadb/rust,mahkoh/rust,quornian/rust,hauleth/rust,aidancully/rust,barosl/rust,dinfuehr/rust,XMPPwocky/rust,aneeshusa/rust,avdi/rust,0x73/rust,robertg/rust,P1start/rust,mdinger/rust,philyoon/rust,mvdnes/rust,miniupnp/rust,zubron/rust,TheNeikos/rust,nwin/rust,carols10cents/rust,zachwick/rust,defuz/rust,richo/rust,miniupnp/rust,defuz/rust,miniupnp/rust,defuz/rust,pshc/rust,krzysz00/rust,aepsil0n/rust,dwillmer/rust,mvdnes/rust,ebfull/rust,emk/rust,mahkoh/rust,gifnksm/rust,P1start/rust,jashank/rust,seanrivera/rust,mahkoh/rust,AerialX/rust-rt-minimal,aepsil0n/rust,emk/rust,kimroen/rust,rohitjoshi/rust,pshc/rust,pelmers/rust,rprichard/rust,philyoon/rust,ruud-v-a/rust,AerialX/rust-rt-minimal,krzysz00/rust,zubron/rust,gifnksm/rust,kimroen/rust,zaeleus/rust,richo/rust,ruud-v-a/rust,vhbit/rust,emk/rust,l0kod/rust,victorvde/rust,ktossell/rust,barosl/rust,ktossell/rust,mdinger/rust,kimroen/rust,mihneadb/rust,0x73/rust,jashank/rust,AerialX/rust,bombless/rust,emk/rust,carols10cents/rust,mdinger/rust,robertg/rust,pshc/rust,bombless/rust,AerialX/rust,jashank/rust,l0kod/rust,ebfull/rust,rprichard/rust,emk/rust,kimroen/rust,reem/rust,zachwick/rust,KokaKiwi/rust,GBGamer/rust,mvdnes/rust,philyoon/rust,AerialX/rust-rt-minimal,kimroen/rust,untitaker/rust,kwantam/rust,andars/rust,bombless/rust,l0kod/rust,emk/rust,aidancully/rust,aidancully/rust,KokaKiwi/rust