Fix configure.ac checking for USE_MD5SUM

'configure.ac' generates 'configure' with invalid syntax,
it is not allowed to test integer like this

Signed-off-by: Jan Havran <havran.jan@email.cz>

configure.ac

@@ -194,7 +194,7 @@ esac
 AC_SUBST(OS_LDFLAGS)
 AC_SUBST(USE_MD5SUM)
 AC_DEFINE_UNQUOTED(USE_MD5SUM,"$USE_MD5SUM",[Using md5sum command line if available])
-AM_CONDITIONAL(USE_MD5SUM, $USE_MD5SUM)
+AM_CONDITIONAL(USE_MD5SUM, [test $USE_MD5SUM = 1])
 
 # system-dependent config.h values
 test "x$JACK_THREAD_STACK_TOUCH" = "x" && JACK_THREAD_STACK_TOUCH=500000