[gtk-doc] configure.ac: use the right quoting for the date check

[Stefan Kost <stefkost src gnome org>]

commit 5bf7cdca8ff16697f9061c695f02293b6b44488d
Author: Stefan Sauer <ensonic users sf net>
Date:   Sun Aug 18 16:09:04 2013 +0200

    configure.ac: use the right quoting for the date check
    
    Fixes one issue in #671519

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 8d25aa3..8d67f7f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -210,7 +210,7 @@ fi
 AC_SUBST(TRACE)
 
 dnl check what date format we can use for the makefile tracing in tests
-date +%s.%N | grep -q "%N"
+date +%s.%N | grep -q '%N'
 if test "$?" == "1"; then
   TS_FMT="+%s.%N"
 else