If OMPI is configured with `--with-hwloc=external` or `--with-hwloc=DIR`
and gfortran is used, I see a lot of warnings when compiling files
under the `ompi/mpi/fortran` directory.
```
f951: Warning: Nonexistent include directory
'BUILD_DIR/opal/mca/hwloc/external/hwloc/include' [-Wmissing-include-dirs]
```
There is no such `include` directory in the source tree and `configure`-
created tree. I think these lines in the `configure.m4` file are wrongly
copied from that for the embedded `hwlocXXX` component in the past.
The `-Wmissing-include-dirs` option is enabled in gfortran by default
but it is not enabled by default (or even with `-Wall`) in gcc and g++.
Signed-off-by: KAWASHIMA Takahiro <t-kawashima@jp.fujitsu.com>
3e179ba95f