Just check for nimdistros instead of adding an additional define.

nimble.nimble

@@ -22,7 +22,7 @@ srcDir = "src"
 
 requires "nim >= 0.13.0", "compiler#head"
 
-when defined(nimbledistros):
+when defined(nimdistros):
   import distros
   if detectOs(Ubuntu):
     foreignDep "libssl-dev"

readme.markdown

@@ -590,7 +590,7 @@ As an example, to specify a dependency on openssl you may put this in your
 .nimble file:
 
 ```nim
-when defined(nimbledistros):
+when defined(nimdistros):
   import distros
   if detectOs(Ubuntu):
     foreignDep "libssl-dev"

src/nimblepkg/nimscriptapi.nim

@@ -22,9 +22,6 @@ 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 +241,6 @@ proc execScript(scriptName: string, flags: StringTableRef,
   defineSymbol("nimscript")
   defineSymbol("nimconfig")
   defineSymbol("nimble")
-  defineSymbol("nimblesupportsdistros")
   registerPass(semPass)
   registerPass(evalPass)