Fix the pkg-config filter

Fix the pkg-config filter

Summary:
I encountered a situation where "pkg-config --libs openssl" included
some linker flags, i.e.

    $ pkg-config --libs openssl
    -Wl,-z,relro -lssl -lcrypto -ldl -lz

This resulted in these linker flags being passed to sed. This change
filters out the linker flags from the pkg-config output.
Closes https://github.com/facebook/folly/pull/559

Reviewed By: pixelb

Differential Revision: D4700120

Pulled By: yfeldblum

fbshipit-source-id: 530d524caf965919418d19a1df53d2b418f67717