Use libc version as a collation version on glibc systems.
Using glibc's version string to detect potential collation definition
changes is not 100% reliable, but it's better than nothing. Currently
this affects only collations explicitly provided by "libc". More work
will be needed to handle the default collation.
Author: Thomas Munro, based on a suggestion from Christoph Berg
Reviewed-by: Peter Eisentraut