Remove trailing comma in configure.ac.

Summary:
- We were inadvertantly emitting a superfluous comma when generating the
configure script, causing it warn on execution. This was caused by
separating two AC_DEFINE() invocations w/ a comma.

Test Plan:
- fbconfig -r folly && fbmake runtests
- ./configure && make check on Ubuntu/FC/Mac

Reviewed By: meyering@fb.com

FB internal diff: D1008924

folly/configure.ac

@@ -84,11 +84,11 @@ AC_COMPILE_IFELSE(
   [AC_LANG_SOURCE[class C { virtual void f() final {} virtual void g() {} };
                   class D : public C { virtual void g() override {} };]],
   [AC_DEFINE([FINAL], [final],
-             [Define to "final" if the compiler supports C++11 "final"]),
+             [Define to "final" if the compiler supports C++11 "final"])
    AC_DEFINE([OVERRIDE], [override],
              [Define to "override" if the compiler supports C++11 "override"])],
   [AC_DEFINE([FINAL], [],
-             [Define to "final" if the compiler supports C++11 "final"]),
+             [Define to "final" if the compiler supports C++11 "final"])
    AC_DEFINE([OVERRIDE], [],
              [Define to "override" if the compiler supports C++11 "override"])]
 )