use parameters instead of globals

move: attrcmp() and attrlookup()
new file ssml.c added to Makefile.am

A corpus of SSML input files will also be needed for fuzzing.
https://github.com/espeak-ng/espeak-ng/issues/407

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.
```

Fixes #401.

The stray space confuses GitHub’s markdown parser.

Based on a patch by Valdis Vitolins <[email protected]>.

Based on a patch by Valdis Vitolins <[email protected]>.