Port to Forte Developer 7 C 5.4 and C99.

Port to Forte Developer 7 C 5.4 and C99.
* src/common.h (add_exclude_tag): Add decl; C99 requires this
and Forte warns about it.
* src/incremen.c: Include <mkdtemp.h> for mkdtemp prototype,
for same reason.
* src/misc.c (get_max_open_files): Rewrite to avoid code that
Forte C complains about as being unreachable.
* src/xheader.c (mtime_code): Rewrite to avoid Forte error
reported by Trond Hasle Amundsen.

* src/incremen.c (compare_dirnames): Rewrite to avoid casts.
* src/utf8.c (string_ascii_p): Likewise.
* src/xheader.c (mtime_coder, volume_size_coder, volume_offset_coder):
Likewise.

SCALAR(0x8283218)