Summary:
The current logic for "which gflags library should we use" checks for
the first library (e.g., libgflags vs. libgflags-shared) listed as a TARGET.
Unfortunately, on some number of systems, the gflags-config.cmake (read:
the file that is supposed to tell you how to use gflags with cmake) actually
defines (but doesn't use!) libraries that don't exist (!) so this logic breaks.

Instead, use the system defined gflags_LIBRARIES variable which explicitly
tells us which library to use.

Reviewed By: yfeldblum, simpkins

Differential Revision: D14512966

fbshipit-source-id: add4ecf6bade502b2d12aad2bdfcf0476eeba465