Like other third-party packages, build the static, but not dynamic gmp libs

GMP seems to be breaking on the cray-prgenv-* builds, and I think that
the attempt to build .so's is the reason.  We could selectively disable
.so's on Crays, but since we disable the .so's in every other third-party
package, why not here as well?

Thomas is going to test this branch to see whether it fixes the GMP
builds on cray-prgenv-*.
Filter the --enable-static/--disable-shared flags only for Cray platforms

This should make the C compile-time warning go away for Mac while
avoding building the .so's on Crays.  It's also most similar to what
we've been testing by default for the past several months.
At Thomas's suggestion, extending the filter to include cray-x*

(in order to avoid disabling for cray-cs, which can probably
better handle the .so's).
Merge pull request #486 from bradcray/build-git-static-only

Build gmp static only on Crays

[co-developed with Thomas]

It turns out that when speculatively building GMP using Cray PrgEnv's
using our existing Makefile logic, we get build errors due to its attempt
to build/test .so files.  Note that for most other third-party packages,
we don't build .so files and that GMP is somewhat of an outlier in this
regard.

For those who track these things, the original proposal was going to be to
disable-shared for all builds, similar to what we do for most other
third-party packages.  The reason we backed away from this and only did it
for Crays was (a) disabling shared resulted in warnings when compiling GMP
programs on Macs and (b) it minimizes the delta for other platforms
relative to what we've been testing for the past several months.