Based on a patch from Hello71 on ticket 20424.

This patch additionally fixes openbsd-malloc support, switches
our tcmalloc support to use pkgconfig, and tells the compiler to
omit system malloc implementations as appropriate.

squash! Add a --with-malloc option.

Edit changelog file to fix a typo and credit Alex Xu in preferred format.

Part of 26979.

Closes 26979.

To avoid future duplicates.

Part of 26979.

Part of 26979.

Fixes bug 26986; bugfix on master.

Part of 26979.

That's what git does for tor.

Part of 26979.

Part of 26979.