Just to close this thread I want to outline what I've done to create my
own rpm of FB 2.1.3 CS with the patch made by Dimitry for bug CORE-2591
integrated.

First I took a look at the ChangeLog file for the upcoming maintenance
release 2.1.4:
http://www.firebirdsql.org/download/snapshot_builds/linux/fb2.1/Changelog_21x.txt

There, searching for the bug-number 2591 I found that Dimitry solved
this issue with a change of file lock.cpp, creating cvs revision
1.129.4.4 of that file.
From the cvs repository I knew that the originial revision of that file
in Firebird release 2.1.3 has been 1.129.4.3, so no other patches will
interfere.

Then I issued the following commands to create the patched rpm:

## Anonymous login to cvs repository:
stefan@pserver:~/tmp> cvs
-d:pserver:anonymous@...:/cvsroot/firebird login
Logging in to
:pserver:anonymous@...:2401/cvsroot/firebird
CVS password: [just confirm with Enter]
## Checkout release 2.1.3 of firebird sources:
stefan@pserver:~/tmp> cvs
-d:pserver:anonymous@...:/cvsroot/firebird co
-r R2_1_3 firebird2
[...]
## Get patched revision 1.129.4.4 of file lock.cpp
stefan@pserver:~/tmp> cvs
-d:pserver:anonymous@...:/cvsroot/firebird
update -r 1.129.4.4 firebird2/src/lock/lock.cpp
P firebird2/src/lock/lock.cpp
stefan@pserver:~/tmp> cd firebird2
## Generated makefiles
stefan@pserver:~/tmp/firebird2> ./autogen.sh --prefix=/opt/firebird
[...]
## make Firebird
stefan@pserver:~/tmp/firebird2> make
[...]
## make rpm and tar.gz
stefan@pserver:~/tmp/firebird2> make dist
[...]
[rpm now in folder ./gen]

The patched version is now running on our test-server. If nothing
happens, I want to install it on our customers site at the end of this week.
I would appreciate any corrections, if I made a mistake in this
instructions.

Stefan