Tweak the ghc-pkg finding code

It now understand the ghc-stage[123] names we use in-tree, and it won't
go looking for any old ghc-pkg if it can't find the one that matches
ghc.

aclocal.m4

@@ -907,24 +907,24 @@ fi
 # FP_PROG_GHC_PKG
 # ----------------
 # Try to find a ghc-pkg matching the ghc mentioned in the environment variable
-# WithGhc. If the latter is unset or no matching ghc-pkg can be found, try to
-# find a plain ghc-pkg. Sets the output variable GhcPkgCmd.
+# WithGhc. Sets the output variable GhcPkgCmd.
 AC_DEFUN([FP_PROG_GHC_PKG],
 [AC_CACHE_CHECK([for ghc-pkg matching $WithGhc], fp_cv_matching_ghc_pkg,
-[fp_ghc_pkg_guess=`echo $WithGhc | sed 's,ghc\(@<:@^/\\@:>@*\)$,ghc-pkg\1,'`
-if "$fp_ghc_pkg_guess" -l > /dev/null 2>&1; then
-  fp_cv_matching_ghc_pkg=$fp_ghc_pkg_guess
-elif "$fp_ghc_pkg_guess" list > /dev/null 2>&1; then
-  # from 6.10, ghc-pkg doesn't support the old -l syntax any more
+[
+# If we are told to use ghc-stage2, then we're using an in-tree
+# compiler. In this case, we just want ghc-pkg, not ghc-pkg-stage2,
+# so we sed off -stage[0-9]$. However, if we are told to use
+# ghc-6.12.1 then we want to use ghc-pkg-6.12.1, so we keep any
+# other suffix.
+fp_ghc_pkg_guess=`echo $WithGhc | sed -e 's/-stage@<:@0-9@:>@$//' -e 's,ghc\(@<:@^/\\@:>@*\)$,ghc-pkg\1,'`
+if "$fp_ghc_pkg_guess" list > /dev/null 2>&1; then
   fp_cv_matching_ghc_pkg=$fp_ghc_pkg_guess
 else
-  fp_cv_matching_ghc_pkg=no
+  AC_MSG_ERROR([Cannot find matching ghc-pkg])
 fi])
-if test x"$fp_cv_matching_ghc_pkg" = xno; then
-  AC_PATH_PROG([GhcPkgCmd], [ghc-pkg])
-else
-  GhcPkgCmd=$fp_cv_matching_ghc_pkg
-fi])# FP_PROG_GHC_PKG
+GhcPkgCmd=$fp_cv_matching_ghc_pkg
+AC_SUBST([GhcPkgCmd])
+])# FP_PROG_GHC_PKG
 
 
 # FP_GCC_EXTRA_FLAGS