Remove unused defined symbols

Remove unused defined symbols

This patch removes unused symbols definitions:
- HAVE_STRCASECMP
- HAVE_STRNCASECMP
- HAVE_STRFTIME
- HAVE_STRTOD
- HAVE_STRTOL
- HAVE_STRTOUL
- HAVE_FMOD
- HAVE_COSH
- HAVE_TANH
- HAVE_SINH
- HAVE_FREXP
- HAVE_MKTIME
- HAVE_MODF
- HAVE_TZNAME

since they aren't used in the current Oniguruma code base. They were
originally mostly part of the build system files where they were defined
based on the system and the presence of the functions strcasecmp,
strncasecmp, strftime, strtod, strtol, strtoul, fmod, cosh, tanh, sinh,
frexp, mktime, modf. Since these functions aren't used in the code the
symbols for checking them can be also removed.

The `HAVE_TZNAME` was part of the AC_STRUCT_TIMEZONE from Autoconf and
other build system parts that checks if struct `tm` has a `tm_zone `
member. This is also not used in the current code base so can be removed.