export El dependency on GMP, MPFR, MPC, QG, and Valgrind include dirs

[evaleev]

No description provided.

[poulson]

Thanks! I am somewhat blindly merging since the tests pass (though I had to restart them due to Travis flakiness).

[evaleev]

Jack, thanks for a super quick turnaround!

This fix should only affect external uses of Elemental that rely on find_package and only if GMP, etc. are in a non-default prefix. So probably all Travis tests will be fine if prereqs are installed via apt-get (i.e. to /usr). Our TA Travis tests were also fine; I only had issues on my Mac where Brew installs GMP & co to /usr/local, and Apple's clang does not look there by default.