adds auto-detection of ALSA config dir

src/PortAudio.jl

@@ -305,7 +305,32 @@ function suppress_err(dofunc::Function)
 end
 
 function __init__()
-    ENV["ALSA_CONFIG_DIR"] = "/usr/share/alsa"
+    envkey = "ALSA_CONFIG_DIR"
+    if envkey ∉ keys(ENV)
+        searchdirs = ["/usr/share/alsa",
+                      "/usr/local/share/alsa",
+                      "/etc/alsa"]
+        confdir_idx = findfirst(searchdirs) do d
+            isfile(joinpath(d, "alsa.conf"))
+        end
+        if confdir_idx === nothing
+            throw(ErrorException(
+                """
+                Could not find ALSA config directory. Searched:
+                $(join(searchdirs, "\n"))
+
+                if ALSA is installed, set the "ALSA_CONFIG_DIR" environment
+                variable. The given directory should have a file "alsa.conf".
+
+                If it would be useful to others, please file an issue at
+                https://github.com/JuliaAudio/PortAudio.jl/issues
+                with your alsa config directory so we can add it to the search
+                paths.
+                """))
+        end
+        confdir = searchdirs[confdir_idx]
+        ENV[envkey] = confdir
+    end
     # initialize PortAudio on module load
     # suppress_err() do
         Pa_Initialize()