pkgconfig complains when the .pc file doesn't have a description. eg:
$ pkg-config Baloo
Package 'Baloo' has no Description: field
With this change, if the DESCRIPTION parameter is not used,
ECM_GENERATE_PKGCONFIG_FILE will :
- First look if there's a metainfo.yaml file and get the description from there
- If the file doesn't exist, or if the description tag is empty or non-existent,
create a description based on the LIB_NAME value