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

folly/m4/fb_filter_pkg_libs.m4

@@ -2,7 +2,7 @@ AC_DEFUN([FB_FILTER_PKG_LIBS],
   [AC_REQUIRE([AC_PROG_SED])
    deps_=`for p in $PKG_DEPS; do pkg-config --libs $p; done`
    filter_=`for l in $deps_;dnl
-     do echo $l | $SED -e 's%\(-l.*\)%-e s/\1//%' -e '/^-L/d';dnl
+     do echo $l | $SED -ne 's%\(-l.*\)%-e s/\1//%p';dnl
      done`
    PKG_LIBS=`echo $1 | $SED $filter_`
   ]