Some PECLs depend on other PECLs and, like internal PHP extension
dependencies, need to be loaded in the correct order. This makes this
possible by adding the argument "peclDeps" to buildPecl, which adds
the extension to buildInputs and is treated the same way as
internalDeps when the extension config is generated.
These packages, and maybe some more include unix.h for some reason.
Creating that file makes them build, and in the case of xdebug also
appear to work.
