message: cxx: Fix library lookup in case pkg_config is installed.
diff: @@ -2101,8 +2101,9 @@ class LibraryConfiguration(drake.Configuration):
  prefix -- Where to find the library.
  token -- Which file to look for (typically, the main header).
  """
  - self.__libs = None
  self.__config = drake.cxx.Config()
  + self.__libs = None
  + self.__prefix = None
  # Search the library with pkg-config
  if prefix is None and PkgConfig.available and name is not None:
  pkg_config = PkgConfig(name)
  @@ -2116,7 +2117,7 @@ class LibraryConfiguration(drake.Configuration):
  self.__libraries_path.append(library_path)
  for library in pkg_config.library:
  self.__config.lib(library)
  - else:
  + if self.__prefix is None:
  # Search the library manually.
  if token is not None:
  include_dir = include_dir or 'include'
  