Fix search name for gflags's .pc file

Fix search name for gflags's .pc file

Summary:
gflags's pkg-config file is installed by default under the name gflags.pc; however, folly has been searching for it under the name libgflags.pc.

Several package managers (Homebrew, apt) install the .pc file under its default upstream name; as such it seems appropriate for folly to look for it under that name, too.

refs Homebrew/homebrew-core#7977.
Closes https://github.com/facebook/folly/pull/531

Reviewed By: yfeldblum

Differential Revision: D4348089

Pulled By: Orvid

fbshipit-source-id: 6adec290ec88b5d3f66103cf8e5094e99f2512ad