Merge pull request #493 from zorun/master

Fix compilation errors for downstream projects caused by incorrect pkconfig paths

pkg-config/jsoncpp.pc.in

 prefix=@CMAKE_INSTALL_PREFIX@
 exec_prefix=${prefix}
-libdir=@LIBRARY_INSTALL_DIR@
-includedir=@INCLUDE_INSTALL_DIR@
+libdir=${exec_prefix}/@LIBRARY_INSTALL_DIR@
+includedir=${prefix}/@INCLUDE_INSTALL_DIR@
 
 Name: jsoncpp
 Description: A C++ library for interacting with JSON