better ext.jl

[vtjnash]

It would be nice if packages ship with a default ext.jl file which just worked if the user already had the dependencies. The hinderance to this appears to be the convoluted way in which the ext.jl file is generated: it uses a macro to do the runtime dlopen check.
https://github.com/JuliaLang/BinDeps.jl/blob/master/src/dependencies.jl#L477

Instead, I propose the following template for ext.jl. ext_paths.jl would be auto-generated and would return the set of paths that BinDeps wants to search:

ext_paths = isfile("ext_paths.jl") ? include("ext_paths.jl") : String[]

checked_lib(path::String) = checked_lib([path])
function checked_lib{T}(path::Vector{T})
  lib = find_library(path, ext_paths)
  lib != "" || error("Unable to load $path\n\nPlease re-run Pkg.build(package), and restart Julia.")
  lib
end
_jl_libpangocairo = checked_lib("libpangocairo-1.0")
_jl_libcairo = checked_lib("libcairo")
_jl_libgobject = checked_lib("libgobject-2.0")
_jl_libpango = checked_lib("libpango-1.0")
#some_lib = checked_lib(["name1", "name2"])

This still assumes that the runtime name/location of the file is mostly constant, but it also allows some runtime determination and checking.

[timholy]

That link to the problematic macro is out of date (not surprising, since this was filed in January). @vtjnash, just so the rest of us can understand, can you edit it using y to get a permalink to a particular version of the file?