these are the only references in the code.

This makes the codebase more consistent. Including speech.h is not necessary anymore in some files. Includes have been removed.

This avoids 'unused parameter' errors in readclause.c.

The only entry point in the file is ProcessSsmlTags().

previous syntax was a quick fix when changing from globals to pointers.

use parameters instead of globals

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

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.

synthesize.h now contains the definitions STRESS_IS_... that should be used with code related to syllable stress.

Note that isBreak and other defines were renumbered so that stress definitions could have values 0-6.

Possible TODOs:
1. Unify with terms used with phonemes, i.e. keywords like isDiminished in compiledata.c and stress_type in phsource/phonemes
2. Add functionality and documentation about STRESS_IS_PRIORITY and STRESS_IS_EMPHASIZED

Stress flag S_2_TO_HEAVY is currently only used by finnish.

Current behaviour skips adding secondary stress if the following syllable is heavy. The behaviour should be to skip adding secondary stress if the rest of the word (excluding last syllable) contains a heavy syllable.

Source of grammar rule and examples of expected behaviour: http://scripta.kotus.fi/visk/sisallys.php?p=13

This reverts commit 0f55204522.

This breaks using voice files like mb-de5-en. It could also use
named values for the stress rules (e.g. 'first-syllable').