Port --enable-gcc-warnings to clang 3.7.0

* configure.ac: Add -Wno-tautological-compare to avoid bogus
warnings about 0 <= rlim.rlim_max.  Remove flags that no longer
seem to be needed, at least in Fedora 23 x86-64.

configure.ac

@@ -986,10 +986,8 @@ AS_IF([test $gl_gcc_warnings = no],
 
   # More things that clang is unduly picky about.
   if test $emacs_cv_clang = yes; then
-    gl_WARN_ADD([-Wno-format-extra-args])
+    gl_WARN_ADD([-Wno-tautological-compare])
     gl_WARN_ADD([-Wno-tautological-constant-out-of-range-compare])
-    gl_WARN_ADD([-Wno-unused-command-line-argument])
-    gl_WARN_ADD([-Wno-unused-value])
   fi
 
   # This causes too much noise in the MinGW build