configure: simplify check for GHC (#475)

This tool lookup and included AM_CONDITIIONAL was originally added in
commit 73b5e40, using the same logic
introduced in this commit. It was originally changed in commit
95558c6 to avoid checking whether GHC
was found, and instead check whether ghc-pkg claimed mmap was found.

A long time later, the use of ghc-pkg was removed in commit
e7a3741, and the check was restored to
"check if GHC exists", which however did not restore the *mechanism*
for checking.

Instead, it attempted to run GHC, whether it existed or not. Checking if
it was found should be enough. But checking that it runs --version is
not particularly useful:
- a command not found / permission denied error is produced if GHC was
  not found, due to trying to execute "" as a command:

  ./configure: 7028: : Permission denied

- if it was found, the version output would be interspersed into
  ./configure output

Similarly a couple lines below, pandoc is checked for and only its
presence is cared about.

configure.ac

@@ -19,7 +19,7 @@ AC_PROG_CXX
 AC_PROG_INSTALL
 AC_PROG_RANLIB
 AC_CHECK_TOOL(GHC, ghc)
-AM_CONDITIONAL([HAVE_GHC], ["$GHC" --version])
+AM_CONDITIONAL([HAVE_GHC], [test "$GHC"])
 AC_CHECK_PROG(PANDOC, pandoc, yes)
 AM_CONDITIONAL([HAVE_PANDOC], [test x"$PANDOC" = x"yes"])
 AM_PROG_AR