list supported triplets in --help

Author: sjkelly

src/AutoBuild.jl

@@ -48,6 +48,8 @@ end
 minimum_compat(compat::String) =
     minimum(VersionNumber(rng.lower.t) for rng in PKG_VERSIONS.semver_spec(compat).ranges)
 
+const PLATFORM_LIST = join(map(triplet, supported_platforms()), "\n    ")
+
 const BUILD_HELP = (
     """
     Usage: build_tarballs.jl [target1,target2,...] [--help]
@@ -116,6 +118,9 @@ const BUILD_HELP = (
         julia build_tarballs.jl x86_64-linux-gnu,i686-linux-gnu
             This builds two tarballs for the two platforms given, with a
             minimum of output messages.
+
+    Supported Platforms:
+        $(PLATFORM_LIST)
     """
 )