automake: Use AM_DISTCHECK_CONFIGURE_FLAGS

Currently we use DISTCHECK_CONFIGURE_FLAGS, which is reserved for
the user. As with other variables, one should use the AM_ variable
within the makefile.

Cc: "10.5" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
diff --git a/Makefile.am b/Makefile.am
index ccf9ac9..f4f0912 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -21,7 +21,7 @@
 
 SUBDIRS = src
 
-DISTCHECK_CONFIGURE_FLAGS = \
+AM_DISTCHECK_CONFIGURE_FLAGS = \
 	--enable-dri3 \
 	--enable-gallium-tests \
 	--enable-gbm \