Build system tweaks

[GuillaumeMercier]

1. It may be interesting to consider using the version of hwloc installed on the target system rather than compiling our own version.
2. libpciaccess is required even though the support for GPU is not activated
3. the whole dependency on libpciaccess can be dropped because hwloc does not really need it (quoting the main hwloc developer)

[samuelkgutierrez]

@GuillaumeMercier, 0ee2e85 address points 2 and 3.

[samuelkgutierrez]

To point 1: version compatibility is challenging enough that we'll stick to our internal version until there is a compelling reason to also support system-installed hwloc libraries.