Adjust `FP_FIND_LIBFFI`

Just set vars, and `AC_SUBST` in top-level configure.

Don't define `HAVE_SYSTEM_LIBFFI` because nothing is using it. It hasn't
be in used since 36093407 (part of the
make build system).

configure.ac

@@ -970,6 +970,8 @@ dnl ################################################################
 
 FP_FIND_LIBFFI
 AC_SUBST(UseSystemLibFFI)
+AC_SUBST(FFILibDir)
+AC_SUBST(FFIIncludeDir)
 
 dnl ** check whether we need -ldl to get dlopen()
 AC_CHECK_LIB([dl], [dlopen], UseLibdl=YES, UseLibdl=NO)

m4/fp_find_libffi.m4

-dnl ** Have libffi?
-dnl --------------------------------------------------------------
-dnl Sets UseSystemLibFFI.
+# FP_FIND_LIBFFI
+# --------------------------------------------------------------
+# Should we used libffi? (yes or no)
+#
+# Sets variables:
+#   - UseSystemLibFFI: [YES|NO]
+#   - FFILibDir: optional path
+#   - FFIIncludeDir: optional path
 AC_DEFUN([FP_FIND_LIBFFI],
 [
   # system libffi
@@ -28,8 +33,6 @@ AC_DEFUN([FP_FIND_LIBFFI],
    fi
   ])
 
-  AC_SUBST(FFIIncludeDir)
-
   AC_ARG_WITH([ffi-libraries],
   [AS_HELP_STRING([--with-ffi-libraries=ARG],
     [Find libffi in ARG [default=system default]])
@@ -42,8 +45,6 @@ AC_DEFUN([FP_FIND_LIBFFI],
    fi
   ])
 
-  AC_SUBST(FFILibDir)
-
   AS_IF([test "$UseSystemLibFFI" = "YES"], [
    CFLAGS2="$CFLAGS"
    CFLAGS="$LIBFFI_CFLAGS $CFLAGS"
@@ -63,7 +64,7 @@ AC_DEFUN([FP_FIND_LIBFFI],
    AC_CHECK_LIB(ffi, ffi_call,
     [AC_CHECK_HEADERS(
       [ffi.h],
-      [AC_DEFINE([HAVE_SYSTEM_LIBFFI], [1], [Define to 1 if you have libffi.])],
+      [],
       [AC_MSG_ERROR([Cannot find ffi.h for system libffi])]
      )],
     [AC_MSG_ERROR([Cannot find system libffi])]

rts/configure.ac

@@ -33,6 +33,10 @@ GHC_CONVERT_PLATFORM_PARTS([host], [Host])
 FPTOOLS_SET_PLATFORM_VARS([host], [Host])
 FPTOOLS_SET_HASKELL_PLATFORM_VARS([Host])
 
+dnl ################################################################
+dnl Check for libraries
+dnl ################################################################
+
 dnl ** check whether we need -ldl to get dlopen()
 AC_CHECK_LIB([dl], [dlopen])
 dnl ** check whether we have dlinfo