[gspell] build: use AM_DISTCHECK_CONFIGURE_FLAGS

[Sébastien Wilmet <swilmet src gnome org>]

commit 33356f4f1a134e961687f31f5e104a3021a58645
Author: Sébastien Wilmet <swilmet gnome org>
Date:   Fri Jul 22 16:56:32 2016 +0200

    build: use AM_DISTCHECK_CONFIGURE_FLAGS
    
    DISTCHECK_CONFIGURE_FLAGS is for users and packagers to override
    distcheck options, while AM_DISTCHECK_CONFIGURE_FLAGS is for the
    maintainer of the software.
    
    https://www.gnu.org/software/automake/manual/automake.html#DISTCHECK_005fCONFIGURE_005fFLAGS
    
    Do the same as in Cheese:
    https://git.gnome.org/browse/cheese/commit/?id=fd4b27876ba079a06cc1e3b7f2957a7771c6dd45

 Makefile.am |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/Makefile.am b/Makefile.am
index b96f434..4760563 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -73,7 +73,7 @@ MAINTAINERCLEANFILES = \
        missing \
        test-driver
 
-DISTCHECK_CONFIGURE_FLAGS = --enable-gtk-doc
+AM_DISTCHECK_CONFIGURE_FLAGS = --enable-gtk-doc
 
 @CODE_COVERAGE_RULES@