Move definition of ONIG_CASE_MAPPING compilation switch outside onigumo files
At r55018, in include/ruby/oniguruma.h, at line 131, I introduced
the compilation switch ONIG_CASE_MAPPING. The purpose of this switch
is to isolate the case-mapping functionality that is introduced into
Ruby but is not used with regular expressions.
The definition on line 131 should be moved outside oniguruma.h,
to a location that makes sure it is seen in oniguruma.h and all files
that include it, but will not be part of backwards pull requests
to onigumo. Such a location may either be an option setting in some
make file or a suitable .h file included into oniguruma.h. I trust
Nobu to find the best location for this.