auto merge of #18194 : jmesmon/rust/libbacktrace-env, r=alexcrichton
auto merge of #18194 : jmesmon/rust/libbacktrace-env, r=alexcrichton

This was run into while attempting to integrate rust with bitbake's build system. I expect it's caused by some other environment variables being set to gcc but didn't track down the exact cause myself.

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