Add RTLD_DEEPBIND existence check (fix builds on Alpine Linux)

[aiwebb]

Closes #187.

Checks for RTLD_DEEPBIND directly, rather than checking for __APPLE__ and __FreeBSD__

This was causing build failures in Alpine Linux, which uses musl instead of glibc and does not include RTLD_DEEPBIND:

https://git.musl-libc.org/cgit/musl/tree/include/dlfcn.h

[pinko-fowle]

This could fix compilation in the official Postgres Docker image. Can we please get this improvement reviewed/merged? @jeevanchalke