Make folly pkg-config more usable on OS X
Summary:
As part of open-sourcing the Watchman C++ client I'm adding an (optional) dependency in Watchman to Folly. When I tried to build this client with Folly on OS X I ran into some problems using the libfolly pkg-config file. This diff addresses these issues:
1. When using `build/bootstrap-osx-homebrew.sh`, the compile config for gflags and OpenSSL are overriden. If present, these custom flags will now be propagated to `libfolly.pc`.
2. I changed the the package requirements in `libfolly.pc` from `Requires.private` to just `Requires`.
Both of these changes are needed as much of Folly's implementation is in header files. As such, consumers of libfolly need to use the same library/include paths when building.
Reviewed By: yfeldblum
Differential Revision:
D4332720
fbshipit-source-id:
6a051ae318ac10bdeb4dc383a16de81f1c0327bb