Keep Curl Bootstrap Working With http:// (#3113)

* Add comments documenting problems in recent versions of NetCDF.
* Add comments to packages required to install curl.

var/spack/repos/builtin/packages/curl/package.py

@@ -30,6 +30,7 @@ class Curl(Package):
     transferring data with URL syntax"""
 
     homepage = "http://curl.haxx.se"
+    # URL must remain http:// so Spack can bootstrap curl
     url      = "http://curl.haxx.se/download/curl-7.46.0.tar.bz2"
 
     version('7.52.1', 'dd014df06ff1d12e173de86873f9f77a')