pgsql: Use libc version as a collation version on glibc systems.

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
Discussion: https://postgr.es/m/4b76c6d4-ae5e-0dc6-7d0d-b5c796a07e34%402ndquadrant.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d5ac14f9ccdda036358c9059d4a29836ebc0c440

Modified Files
--------------
doc/src/sgml/ref/alter_collation.sgml | 10 ++++++++++
src/backend/utils/adt/pg_locale.c     | 14 ++++++++++++--
src/bin/pg_dump/t/002_pg_dump.pl      |  4 ++--
3 files changed, 24 insertions(+), 4 deletions(-)