If ph_code is not located in the phoneme_tab, the resulting ph
value will be NULL. This does not normally happen, but it can
happen if word_phonemes contains garbage data, such as with the
1.49.2 multi-word logic when processing words like 'riposted'.

If the list file contains a text replacement to another
entry in the list file, e.g.:

	ripost     riposte $text
	riposte    rI#p0st

calling it from a prefix or suffix rule such as 'riposted'
causes word_out[0] to be NULL, as TranslateWord3 has the
information needed to perform the mapping. In this case,
no phonemes have been written in this loop and the phonemes
have been calculated, so don't override them.

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