[gtk-mac-integration] Set PREFIX to default_prefix if --prefix isn't set.

[John Ralls <jralls src gnome org>]

commit 38d61ee491996dd8142f6dd4c09a9fc11ab5f9f2
Author: John Ralls <jralls ceridwen us>
Date:   Mon Sep 7 14:18:15 2015 -0700

    Set PREFIX to default_prefix if --prefix isn't set.
    
    autoconf doesn't set prefix to ac_default prefix until AC_OUTPUT and
    that's too late for us.

 configure.ac |    6 +++++-
 1 files changed, 5 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index a9c0286..e30e80c 100644
--- a/configure.ac
+++ b/configure.ac
@@ -86,7 +86,11 @@ GOBJECT_INTROSPECTION_CHECK([0.10.1])
 # flags to aclocal.
 ACLOCAL_AMFLAGS="\${ACLOCAL_FLAGS}"
 AC_SUBST(ACLOCAL_AMFLAGS)
-AC_DEFINE_UNQUOTED([PREFIX],["$prefix"], [Installation prefix (for making filenames)])
+if test "x$prefix" = xNONE; then
+  AC_DEFINE_UNQUOTED([PREFIX],["$ac_default_prefix"], [Installation prefix (for making filenames)])
+else
+  AC_DEFINE_UNQUOTED([PREFIX],["$prefix"], [Installation prefix (for making filenames)])
+fi
 
 AC_CONFIG_FILES(
        Makefile