Use libc version as a collation version on glibc systems.

Using glibc's version number to detect potential collation definition
changes is not 100% reliable, but it's better than nothing.

Author: Thomas Munro
Reviewed-by: Peter Eisentraut
Discussion: https://postgr.es/m/4b76c6d4-ae5e-0dc6-7d0d-b5c796a07e34%402ndquadrant.com

src/backend/utils/adt/pg_locale.c

@@ -70,6 +70,10 @@
 #include <unicode/ucnv.h>
 #endif
 
+#ifdef __GLIBC__
+#include <gnu/libc-version.h>
+#endif
+
 #ifdef WIN32
 /*
  * This Windows file defines StrNCpy. We don't need it here, so we undefine
@@ -1499,7 +1503,7 @@ pg_newlocale_from_collation(Oid collid)
 char *
 get_collation_actual_version(char collprovider, const char *collcollate)
 {
-	char	   *collversion;
+	char	   *collversion = NULL;
 
 #ifdef USE_ICU
 	if (collprovider == COLLPROVIDER_ICU)
@@ -1523,7 +1527,13 @@ get_collation_actual_version(char collprovider, const char *collcollate)
 	}
 	else
 #endif
-		collversion = NULL;
+	if (collprovider == COLLPROVIDER_LIBC)
+	{
+#if defined(__GLIBC__)
+		/* Use the glibc version because we don't have anything better. */
+		collversion = pstrdup(gnu_get_libc_version());
+#endif
+	}
 
 	return collversion;
 }