Before this patch the function was using RZ_PREFIX even when IS_PORTABLE
was set, making the function not working well because RZ_PREFIX might
not exist in the system where the portable build is used (the path was
passed to realpath(), which requires an existing path).
This patch fixes the problem by searching for the BINDIR without using
realpath() on the RZ_PREFIX. Moreover, it avoid retrieving the same
prefix every time by saving it in a global variable.
* Add rz_constructor.h to allow defining constructors/destructors on
multiple compilers.
