Changes

fix --disable-log-macros

Before 9197c1ac, the AC_ARG_ENABLE(log_macros, ...)
action was hardcoded to [log_macros="yes"], so *any*
use of the flag --enable-log-macros or --disable-log-macros
would set log_macros="yes" -> AC_DEFINE([LIBOSMOCORE_NO_LOGGING]).

The commit changed this to the standard [log_macros=$enableval], but
broke the conditional test, so
- AC_ARG_ENABLE sets $enableval to "no" for --disable-* flags
-> log_macros is now "no"
-> test fails
-> LIBOSMOCORE_NO_LOGGING is never defined...

The opposite of what it should do...

Change-Id: I809ab2f61e72428ba21061055296eb83d6d710ab