build: add compatibility symlink for block headers
build: add compatibility symlink for block headers

On FreeBSD and OSX, the block runtime functions are defined on a Block.h
and a Block_private.h headers on the include path as opposed to
libobjc2's objc/blocks_runtime.h and objc/blocks_private.h respectively.

This commit amends libobjc2's install routine to create the
FreeBSD-style headers as symlinks to the ones provided by libobjc2 for
improved compatibility.

gnustep/libobjc2,gnustep/libobjc2