Make `Pkg.build` error message clearer.
Merge pull request #9158 from lucasb-eyer/pkgbuild-error

Make `Pkg.build` error message clearer.
Merge branch 'master' of https://github.com/JuliaLang/julia
Revert "Add tips about working with LLVM."

This reverts commit b7626964794261fa2c6616392db2ad93f67c439c.