This can be more efficient than using strstr/Strstr with a single
character search string.

GCC is able to optimise strstr() with a single character literal
search string to strchr(), but clang doesn't, and likely no compiler
can for Strstr() (unless some sort of inter-object optimisation such
as LTO is used) since the literal string is in a different source file
to the strstr() call.

Previously they were silently ignored in this context (but #if defined
already worked here if you need a workaround which works for older
versions).

Fixes #2183

We need to insert newlines for a multi-line macro invocation, and
we shouldn't leak memory.

Swig_string_mangle      => Swig_name_mangle_string
Swig_name_mangle        => Swig_name_mangle_string
Swig_string_mangle_type => Swig_name_mangle_type

Remove some erroneously added brackets_increment() calls.

Reject <=> in preprocessor expressions with a clear error message (it
seems it isn't supported here - clang and gcc don't at least).

The type returned by `<=>` is not `bool`.  We pretend it's
`int` for now, which should work for how it's likely to be used
in constant expressions.

Fixes #1622