Fix syntax issue in configure script

The change is not done in configure.ac, because the package doesn't
autoreconf properly.

Patch on configure.ac submitted upstream at
https://bugs.freedesktop.org/show_bug.cgi?id=90292.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/configure
===================================================================
--- a/configure
+++ b/configure
@@ -19738,7 +19738,7 @@
 CXXFLAGS="$CXXFLAGS $LIBTIFF_CFLAGS"
 LIBS="$LIBS $LIBTIFF_LIBS"
 if test x$enable_libtiff = xyes; then
-  if test x"$LIBTIFF_LIBS" != ; then
+  if test x"$LIBTIFF_LIBS" != x; then
     ac_fn_cxx_check_func "$LINENO" "TIFFOpen" "ac_cv_func_TIFFOpen"
 if test "x$ac_cv_func_TIFFOpen" = xyes; then :