build-sys: pass originally configured --enable-split-usr to distcheck

Previously we always ran distcheck with --disable-split-usr. This caused
test-path-util to fail with

  Assertion 'fsck_exists("minix") == 0' failed at ../src/test/test-path-util.c:224, function test_fsck_exists(). Aborting.

as looking up fsck.minix would only look into DEFAULT_PATH_NORMAL, but on these
systems fsck is in /sbin/.

Makefile.am

@@ -6799,7 +6799,6 @@ DISTCHECK_CONFIGURE_FLAGS = \
 	--with-pamlibdir=$$dc_install_base/$(pamlibdir) \
 	--with-pamconfdir=$$dc_install_base/$(pamconfdir) \
 	--with-rootprefix=$$dc_install_base \
-	--disable-split-usr \
 	--enable-kdbus \
 	--enable-compat-libs
 
@@ -6823,6 +6822,14 @@ DISTCHECK_CONFIGURE_FLAGS += \
 	--enable-gtk-doc
 endif
 
+if ENABLE_SPLIT_USR
+DISTCHECK_CONFIGURE_FLAGS += \
+	--enable-split-usr
+else
+DISTCHECK_CONFIGURE_FLAGS += \
+	--disable-split-usr
+endif
+
 #
 # Require python when making dist
 #

configure.ac

@@ -1433,6 +1433,7 @@ AC_SUBST(DEFAULT_DKR_INDEX_URL)
 AS_IF([test "x${enable_split_usr}" = "xyes"], [
         AC_DEFINE(HAVE_SPLIT_USR, 1, [Define if /bin, /sbin aren't symlinks into /usr])
 ])
+AM_CONDITIONAL(ENABLE_SPLIT_USR, [test "x${enable_split_usr}" = "xyes"])
 
 # Work around intltoolize and gtk-doc problems in VPATH builds
 AM_CONDITIONAL([ENABLE_GTK_DOC_TESTS], [test "x$0" = "x./configure"],