configure: fix fuse errors

Fix the following syntax errors:
  ...
 checking for fuse... yes
 ./configure: line 5243: test: =: unary operator expected
 checking for pthread_create in -lpthread... yes
 checking that generated files are newer than configure... done
 configure: creating ./config.status
  ...

configure.ac

@@ -86,12 +86,12 @@ AS_IF([test "x$build_fuse" = "xyes"], [
     AC_CHECK_LIB(cuse, cuse_dev_create)
   else
     PKG_CHECK_MODULES(fuse, fuse, [], PKG_CHECK_MODULES(fuse, fuse3, [use_fuse3=yes], [AC_MSG_ERROR([Can't find fuse])]))
-  if test $use_fuse3 = yes; then
+  fi
+  if test "x$use_fuse3" = yes; then
     AC_SUBST(CPPFLAGS, "$CPPFLAGS -DFUSE_USE_VERSION=30")
   else
     AC_SUBST(CPPFLAGS, "$CPPFLAGS -DFUSE_USE_VERSION=29")
   fi
-fi
 ])
 
 if test $backend = freebsd; then