Get rid of re_set_syntax

Get rid of re_set_syntax

Instead of using a global variable for storing regex syntax, pass it
to re_compile_pattern.  This is only enabled when compiling Emacs (i.e.
‘#ifdef emacs’).

* src/regex.h (re_set_syntax): Declare only #ifndef emacs.
(re_compile_pattern): Now takes syntax argument #ifdef emacs.

* src/regex.c (re_syntax_options): Define only #ifndef emacs.
(re_compile_pattern): Use the new syntax argument #ifdef emacs.

* src/search.c (compile_pattern_1): Don’t use re_set_syntax and
instead pass syntax to re_compile_pattern directly.