Fix --with-compiler failing to locate compiler on Windows

Due to idiosyncrasies (courtesy of the `SearchPath` Win32 function) in
the Windows implementation of `findExecutable` we need to explicitly try
both, with and without appending the Windows specific executable
extensions (i.e. `.exe`).

For example, before this patch,

   cabal new-build -w ghc-8.6.2

would fail to find the `ghc-8.6.2.exe` even though it's on `%PATH%` and
invoking

   ghc-8.6.2 --version

on the `CMD.exe` shell would have been able to locate the executable.

Author: hvr

Cabal/Distribution/Simple/Program/Find.hs

@@ -102,7 +102,7 @@ findProgramOnSearchPath verbosity searchpath prog = do
     -- On windows, getSystemSearchPath is not guaranteed 100% correct so we
     -- use findExecutable and then approximate the not-found-at locations.
     tryPathElem ProgramSearchPathDefault | buildOS == Windows = do
-      mExe    <- findExecutable prog
+      mExe    <- firstJustM [ findExecutable (prog <.> ext) | ext <- exeExtensions ]
       syspath <- getSystemSearchPath
       case mExe of
         Nothing ->
@@ -130,6 +130,15 @@ findProgramOnSearchPath verbosity searchpath prog = do
             then return (Just f, reverse fs')
             else go (f:fs') fs
 
+    -- Helper for evaluating actions until the first one returns 'Just'
+    firstJustM :: Monad m => [m (Maybe a)] -> m (Maybe a)
+    firstJustM [] = return Nothing
+    firstJustM (ma:mas) = do
+      a <- ma
+      case a of
+        Just _  -> return a
+        Nothing -> firstJustM mas
+
 -- | Interpret a 'ProgramSearchPath' to construct a new @$PATH@ env var.
 -- Note that this is close but not perfect because on Windows the search
 -- algorithm looks at more than just the @%PATH%@.