Auto merge of #19739 - gootorov:master, r=jdm
Auto merge of #19739 - gootorov:master, r=jdm

Add note to Gentoo setup to set LIBCLANG_PATH envvar

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Without `LIBCLANG_PATH` environment variable set the build for `gecko-media` fails as in https://github.com/servo/servo/issues/19351 for Gentoo Linux. This patch adds a note to the README to set the environment variable.

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cbrewster/servo,peterjoel/servo,avadacatavra/servo,ConnorGBrewster/servo,DominoTree/servo,KiChjang/servo,sadmansk/servo,notriddle/servo,thiagopnts/servo,larsbergstrom/servo,mbrubeck/servo,larsbergstrom/servo,pyfisch/servo,peterjoel/servo,fiji-flo/servo,emilio/servo,SimonSapin/servo,notriddle/servo,splav/servo,nnethercote/servo,mbrubeck/servo,szeged/servo,larsbergstrom/servo,saneyuki/servo,DominoTree/servo,KiChjang/servo,canaltinova/servo,canaltinova/servo,fiji-flo/servo,cbrewster/servo,emilio/servo,larsbergstrom/servo,splav/servo,upsuper/servo,fiji-flo/servo,avadacatavra/servo,szeged/servo,paulrouget/servo,splav/servo,szeged/servo,mattnenterprise/servo,emilio/servo,cbrewster/servo,cbrewster/servo,KiChjang/servo,pyfisch/servo,jimberlage/servo,peterjoel/servo,SimonSapin/servo,DominoTree/servo,canaltinova/servo,DominoTree/servo,canaltinova/servo,splav/servo,nnethercote/servo,larsbergstrom/servo,saneyuki/servo,dati91/servo,jimberlage/servo,mbrubeck/servo,larsbergstrom/servo,mbrubeck/servo,thiagopnts/servo,cbrewster/servo,larsbergstrom/servo,mattnenterprise/servo,notriddle/servo,peterjoel/servo,KiChjang/servo,SimonSapin/servo,emilio/servo,splav/servo,emilio/servo,dati91/servo,paulrouget/servo,saneyuki/servo,thiagopnts/servo,anthgur/servo,notriddle/servo,jimberlage/servo,peterjoel/servo,szeged/servo,KiChjang/servo,szeged/servo,splav/servo,mattnenterprise/servo,dati91/servo,saneyuki/servo,szeged/servo,mbrubeck/servo,saneyuki/servo,avadacatavra/servo,peterjoel/servo,ConnorGBrewster/servo,DominoTree/servo,avadacatavra/servo,sadmansk/servo,mattnenterprise/servo,szeged/servo,larsbergstrom/servo,avadacatavra/servo,upsuper/servo,avadacatavra/servo,dati91/servo,saneyuki/servo,jimberlage/servo,notriddle/servo,thiagopnts/servo,nnethercote/servo,larsbergstrom/servo,emilio/servo,KiChjang/servo,notriddle/servo,KiChjang/servo,nnethercote/servo,anthgur/servo,thiagopnts/servo,mbrubeck/servo,upsuper/servo,notriddle/servo,DominoTree/servo,canaltinova/servo,saneyuki/servo,ConnorGBrewster/servo,cbrewster/servo,upsuper/servo,emilio/servo,anthgur/servo,pyfisch/servo,emilio/servo,jimberlage/servo,nnethercote/servo,paulrouget/servo,fiji-flo/servo,jimberlage/servo,splav/servo,szeged/servo,avadacatavra/servo,fiji-flo/servo,ConnorGBrewster/servo,jimberlage/servo,peterjoel/servo,paulrouget/servo,nnethercote/servo,peterjoel/servo,sadmansk/servo,paulrouget/servo,SimonSapin/servo,dati91/servo,sadmansk/servo,canaltinova/servo,upsuper/servo,splav/servo,paulrouget/servo,mattnenterprise/servo,canaltinova/servo,SimonSapin/servo,sadmansk/servo,notriddle/servo,emilio/servo,DominoTree/servo,szeged/servo,SimonSapin/servo,upsuper/servo,notriddle/servo,emilio/servo,upsuper/servo,paulrouget/servo,pyfisch/servo,peterjoel/servo,nnethercote/servo,avadacatavra/servo,anthgur/servo,larsbergstrom/servo,mattnenterprise/servo,anthgur/servo,dati91/servo,thiagopnts/servo,notriddle/servo,upsuper/servo,thiagopnts/servo,splav/servo,mattnenterprise/servo,paulrouget/servo,anthgur/servo,jimberlage/servo,pyfisch/servo,pyfisch/servo,thiagopnts/servo,dati91/servo,cbrewster/servo,peterjoel/servo,paulrouget/servo,nnethercote/servo,mattnenterprise/servo,saneyuki/servo,DominoTree/servo,paulrouget/servo,jimberlage/servo,nnethercote/servo,KiChjang/servo,szeged/servo,mbrubeck/servo,sadmansk/servo,ConnorGBrewster/servo,dati91/servo,pyfisch/servo,fiji-flo/servo,SimonSapin/servo,anthgur/servo,saneyuki/servo,KiChjang/servo,sadmansk/servo,sadmansk/servo,canaltinova/servo,ConnorGBrewster/servo,pyfisch/servo,cbrewster/servo,ConnorGBrewster/servo,nnethercote/servo,mbrubeck/servo,saneyuki/servo,SimonSapin/servo,DominoTree/servo,anthgur/servo,splav/servo,pyfisch/servo,DominoTree/servo,KiChjang/servo,pyfisch/servo,fiji-flo/servo,ConnorGBrewster/servo,fiji-flo/servo