travis: configure build with dependency tracking disabled

Dependency tracking is completely useless for one-time builds,
so configure build with dependency tracking disabled.

* travis-build.sh (DISTCHECK_CONFIGURE_FLAGS): Add
--disable-dependency-tracking.

travis-build.sh

@@ -1,5 +1,8 @@
 #!/bin/sh -ex
 
+DISTCHECK_CONFIGURE_FLAGS='--disable-dependency-tracking'
+export DISTCHECK_CONFIGURE_FLAGS
+
 case "$CC" in
 	gcc*)
 		DISTCHECK_CONFIGURE_FLAGS="$DISTCHECK_CONFIGURE_FLAGS --enable-gcc-Werror"
@@ -52,9 +55,6 @@ echo 'END OF BUILD ENVIRONMENT INFORMATION'
 
 export CC_FOR_BUILD="$CC"
 
-[ -z "${DISTCHECK_CONFIGURE_FLAGS-}" ] ||
-	export DISTCHECK_CONFIGURE_FLAGS
-
 ./git-set-file-times
 ./bootstrap
 ./configure --enable-maintainer-mode \