Fix paths in pkg-config file (#556)

pkg-config should be giving absolute paths rather than relative paths.

yaml-cpp.pc.cmake

-libdir=@LIB_INSTALL_DIR@
-includedir=@INCLUDE_INSTALL_ROOT_DIR@
+prefix=@CMAKE_INSTALL_PREFIX@
+exec_prefix=${prefix}
+includedir=${prefix}/@INCLUDE_INSTALL_ROOT_DIR@
+libdir=${exec_prefix}/@LIB_INSTALL_DIR@
 
 Name: Yaml-cpp
 Description: A YAML parser and emitter for C++