Handle postgresql and sqlite3 libs properly

Do not fail to compile when one of them is missing but the other one is not.

Raise an error when trying to open a database with the missing library.

see #3237

src/database/postgresql_engine.cpp

@@ -1,3 +1,6 @@
+#include <biboumi.h>
+#ifdef PQ_FOUND
+
 #include <database/postgresql_engine.hpp>
 
 #include <database/postgresql_statement.hpp>
@@ -75,3 +78,5 @@ std::string PostgresqlEngine::id_column_type()
 {
   return "SERIAL";
 }
+
+#endif