A nonexistent directory in PKG_CONFIG_PATH should not be an error

pkg-config itself does not consider it an error if a directory that is
listed in PKG_CONFIG_PATH does not exist. Instead of raising an
exception if a directory does not exist, simply don't search it.
Merge pull request #9 from lpsinger/no_more_not_a_directory_error

A nonexistent directory in PKG_CONFIG_PATH should not be an error