configuration: OMPI v2.x with external hwloc from the distro

[artpol84]

@dannyauble reported the problem with OMPI configuration.

In the target environment hwloc 1.11.3-2 was installed from the distro package in the default location in /usr/

For the case where we configured without parameters:

./configure ... --with-hwloc ...

not all hwloc paths was properly set and compilation failed.
@dannyauble noticed that MCA_hwloc_external_header was:

#define MCA_hwloc_external_header "hwloc.h"

instead of

#define MCA_hwloc_external_header "/usr/include/hwloc.h"

Problem can be solved by setting full path to hwloc at config time:

./configure ... --with-hwloc=/usr/ ...

[jsquyres]

This is likely related to #2616.

[ggouaillardet]

@artpol84 basically, the issue is we have a hwloc.h file in opal/mca/hwloc/hwloc.h and we automatically add -Iopal/mca/hwloc in the CPPFLAGS.
bottom line, if we want the external hwloc.h, we cannot simply

#include <hwloc.h>

but we have to

#include "/usr/include/hwloc.h"

(well, we do that via the ```MCA_hwloc_external_header" macro)

since the MCA framework is called hwloc, then the header file must be named hwloc.h.
imho, in order to correctly address this issue, the easiest way is to rename the hwloc framework
(hwtopo ?). unfortunatly, this is also the longest path ...

[artpol84]

@jsquyres I think you are right. empty argument or external has same meaning. Closing this one.