I'm trying to install mpich-3.0rc1 on an ancient (RHEL4) machine and get this error. Making all in tools/topo/hwloc/hwloc make[3]: Entering directory `/work/parmod/mpich-3.0rc1/src/pm/hydra/tools/topo/hwloc/hwloc' Making all in src make[4]: Entering directory `/work/parmod/mpich-3.0rc1/src/pm/hydra/tools/topo/hwloc/hwloc/src' /bin/sh ../../../../../libtool --tag=CC --mode=compile gcc -DHAVE_CONFIG_H -I. -I../../../../../include -I../../../../../tools/topo/hwloc/hwloc/include/private/autogen -I../../../../../tools/topo/hwloc/hwloc/include/hwloc/autogen -I/work/parmod/mpich-3.0rc1/src/pm/hydra/tools/topo/hwloc/hwloc/include -I/work/parmod/mpich-3.0rc1/src/pm/hydra/tools/topo/hwloc/hwloc/include -DHWLOC_INSIDE_LIBHWLOC -I/work/parmod/mpich-3.0rc1/src/pm/hydra/tools/topo/hwloc/hwloc/include -I/work/parmod/mpich-3.0rc1/src/pm/hydra/tools/topo/hwloc/hwloc/include -I/work/parmod/mpich-3.0rc1/src/mpl/include -I/work/parmod/mpich-3.0rc1/src/mpl/include -I/work/parmod/mpich-3.0rc1/src/openpa/src -I/work/parmod/mpich-3.0rc1/src/openpa/src -I/work/parmod/mpich-3.0rc1/src/mpi/romio/include -fvisibility=hidden -I/usr/include/libxml2 -fvisibility=hidden -I/usr/include/libxml2 -O2 -MT traversal.lo -MD -MP -MF .deps/traversal.Tpo -c -o traversal.lo traversal.c libtool: compile: gcc -DHAVE_CONFIG_H -I. -I../../../../../include -I../../../../../tools/topo/hwloc/hwloc/include/private/autogen -I../../../../../tools/topo/hwloc/hwloc/include/hwloc/autogen -I/work/parmod/mpich-3.0rc1/src/pm/hydra/tools/topo/hwloc/hwloc/include -I/work/parmod/mpich-3.0rc1/src/pm/hydra/tools/topo/hwloc/hwloc/include -DHWLOC_INSIDE_LIBHWLOC -I/work/parmod/mpich-3.0rc1/src/pm/hydra/tools/topo/hwloc/hwloc/include -I/work/parmod/mpich-3.0rc1/src/pm/hydra/tools/topo/hwloc/hwloc/include -I/work/parmod/mpich-3.0rc1/src/mpl/include -I/work/parmod/mpich-3.0rc1/src/mpl/include -I/work/parmod/mpich-3.0rc1/src/openpa/src -I/work/parmod/mpich-3.0rc1/src/openpa/src -I/work/parmod/mpich-3.0rc1/src/mpi/romio/include -fvisibility=hidden -I/usr/include/libxml2 -fvisibility=hidden -I/usr/include/libxml2 -O2 -MT traversal.lo -MD -MP -MF .deps/traversal.Tpo -c traversal.c -fPIC -DPIC -o .libs/traversal.o In file included from traversal.c:12: /work/parmod/mpich-3.0rc1/src/pm/hydra/tools/topo/hwloc/hwloc/include/private/misc.h:41:2: error: #error "unknown size for unsigned int." make[4]: *** [traversal.lo] Error 1 make[4]: Leaving directory `/work/parmod/mpich-3.0rc1/src/pm/hydra/tools/topo/hwloc/hwloc/src' make[3]: *** [all-recursive] Error 1 make[3]: Leaving directory `/work/parmod/mpich-3.0rc1/src/pm/hydra/tools/topo/hwloc/hwloc' make[2]: *** [all-recursive] Error 1 make[2]: Leaving directory `/work/parmod/mpich-3.0rc1/src/pm/hydra' make[1]: *** [all-recursive] Error 1 make[1]: Leaving directory `/work/parmod/mpich-3.0rc1' make: *** [all] Error 2 It wasn't obvious to me from the logs why this was being tripped. Configure log attached.