Add explicit visibility flags to override {fmt} defaults

[klevzoff]

This PR explicitly overrides options in {fmt}'s CMake that add -fvisibility=hidden compiler flag and result in linker warnings on MacOS.

@herve-gross @CusiniM Could one of you test this branch on your Macs please? (I know I can do it on Travis, but I don't have a GEOSX PR open atm). Do something like

cd thirdPartyLibs/build-darwin-clang-release
git fetch origin
git checkout bugfix/klevzoff/fmt-visibility
rm -rf fmt
cmake ..
make fmt

and then build GEOSX and see if the ld warning reappears.