== Short version
Do not export special variables into the environment (e.g., LIBS,
LDFLAGS, etc.) when invoking subdir configure scripts. This prevents
problems described in open-mpi/ompi#471.
== More detail
Exporing special env variables before invoking a subdir configure
script causes problems in some cases. E.g., in open-mpi/ompi#471,
when the user configures with `--with-hwloc=/path/to/hwloc`, and that
directory is *not* in a default linker search location will cause the
libevent subdir configuration to fail.
This happens because:
1. We'll pass LIBS="-L/path/to/hwloc/lib -lhwloc" to the libevent
configure script
1. Meaning: configure-generated executables will link successfully
1. But unless LD_LIBRARY_PATH (or some other
tell-the-linker-where-to-find-things mechanism) includes
/path/to/hwloc/lib, the executable can't run.
Specifically, the libevent "hey, does the compiler generate proper
executables?" check will fail, and configure will abort (because OMPI
needs libevent).
I checked the history: exporting these vars dates all the way back to
LAM/MPI. I can't think of a reason why we need to export these
variables -- AC_CONFIG_SUBDIRs doesn't do it; subdir configure scripts
should be orthogonal from the upper-layer configure script (and its
variables). So let's remove these export statements and see if
anything breaks.
