More intelligent definition of nimbledistros.

This fixes issues with using latest Nimble in an
0.15.2 environment.

Squashed commit of the following:

commit 4c5f791873b7965a25b9dbdb29a96b38a065c478
Author: Dominik Picheta <dominikpicheta@gmail.com>
Date:   Tue Jan 3 18:41:30 2017 +0000

    Disable output in tester.

commit 9f4af9685a36fbebc5a71b6d39130c2d2b30bc05
Author: Dominik Picheta <dominikpicheta@gmail.com>
Date:   Tue Jan 3 18:30:38 2017 +0000

    Attempt at fixing tests.

commit d0de031d1ce11be0f106eb6d92885b6833ce95b0
Author: Dominik Picheta <dominikpicheta@gmail.com>
Date:   Tue Jan 3 18:16:40 2017 +0000

    Testing.

src/nimblepkg/nimscriptapi.nim

@@ -22,6 +22,9 @@ var
   foreignDeps*: seq[string] = @[] ## The foreign dependencies. Only
                                   ## exported for 'distros.nim'.
 
+when defined(nimdistros) and defined(nimblesupportsdistros):
+  const nimbledistros = true
+
 proc requires*(deps: varargs[string]) =
   ## Call this to set the list of requirements of your Nimble
   ## package.

src/nimblepkg/nimscriptsupport.nim

@@ -241,7 +241,7 @@ proc execScript(scriptName: string, flags: StringTableRef,
   defineSymbol("nimscript")
   defineSymbol("nimconfig")
   defineSymbol("nimble")
-  defineSymbol("nimbledistros")
+  defineSymbol("nimblesupportsdistros")
   registerPass(semPass)
   registerPass(evalPass)