configure: disable -Wunused-const-variable

The option became too aggressive with GCC 6, generating nearly 500
warnings from static const variables defined in assorted headers

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: James Almer <jamrial@gmail.com>

configure

@@ -5780,6 +5780,7 @@ check_disable_warning -Wno-parentheses
 check_disable_warning -Wno-switch
 check_disable_warning -Wno-format-zero-length
 check_disable_warning -Wno-pointer-sign
+check_disable_warning -Wno-unused-const-variable
 
 # add some linker flags
 check_ldflags -Wl,--warn-common