New method for checking for presence of tex program.

Now instead of using `findExecutable`, which has limitations
on Windows, we just do `progname --version` and see if it
returns successfully.  Closes #2903.

pandoc.cabal

@@ -424,7 +424,8 @@ Executable pandoc
                  aeson >= 0.7.0.5 && < 0.12,
                  yaml >= 0.8.8.2 && < 0.9,
                  containers >= 0.1 && < 0.6,
-                 HTTP >= 4000.0.5 && < 4000.4
+                 HTTP >= 4000.0.5 && < 4000.4,
+                 process >= 1.0 && < 1.5
   if flag(network-uri)
      Build-Depends: network-uri >= 2.6 && < 2.7, network >= 2.6
   else

pandoc.hs

@@ -52,6 +52,7 @@ import Data.Char ( toLower, toUpper )
 import Data.List ( delete, intercalate, isPrefixOf, isSuffixOf, sort )
 import System.Directory ( getAppUserDataDirectory, findExecutable,
                           doesFileExist, Permissions(..), getPermissions )
+import System.Process ( readProcessWithExitCode )
 import System.IO ( stdout, stderr )
 import System.IO.Error ( isDoesNotExistError )
 import qualified Control.Exception as E
@@ -1401,8 +1402,8 @@ convertWithOpts opts args = do
                               _ | html5Output   -> "wkhtmltopdf"
                               _                 -> latexEngine
               -- check for pdf creating program
-              mbPdfProg <- findExecutable pdfprog
-              when (isNothing mbPdfProg) $
+              (ec,_,_) <- readProcessWithExitCode pdfprog ["--version"] ""
+              when (ec /= ExitSuccess) $
                    err 41 $ pdfprog ++ " not found. " ++
                      pdfprog ++ " is needed for pdf output."