Add --with-mangling.

Add the --with-mangling configure option, which can be used to specify
name mangling on a per public symbol basis that takes precedence over
--with-jemalloc-prefix.

Expose the memalign() and valloc() overrides even if
--with-jemalloc-prefix is specified.  This change does no real harm, and
simplifies the code.
15 files changed