Summary:
If the system has gflags with both `google` and `gflags` namespaces installed, we try to define GFLAGS as two things. This breaks the compile.
Fix: Use `else if` -- try compiling with `google` namespace only if compile with `gflags` failed
Test Plan: build_tools/build_detect_platform correctly identifies gflags
Reviewers: lgalanis
Reviewed By: lgalanis
Subscribers: dhruba, leveldb
Differential Revision: https://reviews.facebook.net/D37389
742fa9e316