Remove linking of external libraries from pkg-config.

Author: Jan Vcelak <jvcelak@redhat.com>

diff -u a/pion-net.pc.in b/pion-net.pc.in
--- a/pion-net.pc.in
+++ b/pion-net.pc.in
@@ -10,6 +10,6 @@ datadir=@datarootdir@
 Name: pion-net
 Description: a C++ framework for building lightweight HTTP interfaces
 Version: @VERSION@
-#Requires: boost-1.34.1 boost-asio-0.3.9
-Libs: @LDFLAGS@ @PION_EXTERNAL_LIBS@ -L${libdir} -lpion-common -lpion-net
-Cflags: @CPPFLAGS@ -I${includedir}
+Libs: -L${libdir} -lpion-common -lpion-net
+Libs.private: @PION_EXTERNAL_LIBS@
+Cflags: -I${includedir}