Before this change, the clang compiler produced the following warning:

```
In file included from src/libespeak-ng/espeak_api.c:35:
src/libespeak-ng/event.h:65:28: warning: declaration of 'struct timespec' will
      not be visible outside of this function [-Wvisibility]
void clock_gettime2(struct timespec *ts);
                           ^
src/libespeak-ng/event.h:66:28: warning: declaration of 'struct timespec' will
      not be visible outside of this function [-Wvisibility]
void add_time_in_ms(struct timespec *ts, int time_in_ms);
                           ^
2 warnings generated.
```

The libespeak files are supposed to be C code, so use C linkage
for all the functions defined in that code when they are being
referenced in the C++ code (i.e. espeakedit).

espeak_SetSynthCallback() can be called either before or after espeak_Initialize().


git-svn-id: https://espeak.svn.sourceforge.net/svnroot/espeak/trunk@31 d46cf337-b52f-0410-862d-fd96e6ae7743