Add interface for changing ggwave's internal logging (#52)

* ggwave : add interface for changing ggwave's internal logging

Using GGWave::setLogFile() it is now possible to change the log file
used internally by ggwave, or disable it all together.

* ggwave : add comments about thread-safety of setLogFile

tests/test-ggwave.c

@@ -14,6 +14,9 @@
 #define CHECK_F(cond) CHECK(!(cond))
 
 int main() {
+    //ggwave_setLogFile(NULL); // disable logging
+    ggwave_setLogFile(stdout);
+
     ggwave_Parameters parameters = ggwave_getDefaultParameters();
     parameters.sampleFormatInp = GGWAVE_SAMPLE_FORMAT_I16;
     parameters.sampleFormatOut = GGWAVE_SAMPLE_FORMAT_I16;