option to only build static libs

[adammoody]

This is work in progress. It defines BUILD_SHARED_LIBS as an option, which if disabled, passes --disabled-shared to the lwgrp and dtcmp configure line to disable shared library builds of those.

We'd need to similarly update the other components perhaps by dropping SHARED from their ADD_LIBRARY statements.

A quick test with SCR's cmake shows we'd also need changes to the builds of the example programs.

cmake -DCMAKE_INSTALL_PREFIX=../install -DSCR_RESOURCE_MANAGER=SLURM -DBUILD_SHARED_LIBS=OFF -DSCR_LINK_STATIC=ON ../
make install
<snip>
[ 89%] Building C object examples/CMakeFiles/test_api.dir/test_api.c.o
[ 90%] Linking C executable test_api
scr-top-v3.0rc1/install/lib/libdtcmp.a(libdtcmp_la-dtcmp_sortv_cheng.o): In function `compute_weighted_median':
scr-top-v3.0rc1/dtcmp/src/dtcmp_sortv_cheng.c:82: undefined reference to `lwgrp_comm_size'
scr-top-v3.0rc1/dtcmp/src/dtcmp_sortv_cheng.c:94: undefined reference to `lwgrp_comm_alltoall'

I think we'd at least need to add a -llwgrp, but to do that, we also need a Find_Package(LWGRP). Not sure how many dominoes might fall chasing this path.

[adammoody]

This is related to: LLNL/scr#322

@gonsie , would you please take a look over this when you get a chance? I think it's settled out for now, though any feedback regarding the above PR might apply here, too.