improvements to "make dbschema" and "make dist"

(1) File "dbschema.c" should depend on "schema.sql". This would help us avoid issues like ZBX-3736.

(2) Files "ibm_db2.sql", "mysql.sql", "oracle.sql", "postgresql.sql", "sqlite3.sql" should also depend on "schema.sql".

(3) It should be possible to create a distribution from a distribution (i.e., run "make dist" successfully in the distribution, created by "make dist").

The fix is partially inhibited by the fact that it is not clear whether we want to include "gen.pl" and "schema.sql" in the distribution. If not, then "dbschema.c" cannot depend on "schema.sql" unconditionally (because "make dbschema.c" would fail in the distribution directory). If yes, then it makes our distribution heavier and makes files "ibm_db2.sql", "mysql.sql", "oracle.sql", "postgresql.sql", "sqlite3.sql" unnecessary in the distribution.