Update sp_mgr.c fix issue #103

[arpadffy]

Issue #103 describes the problem
The environment:

SYSTEM@x86vms$ mms /ident
%MMS-I-IDENT, MMS V4.0-4 ▒ Copyright 2022 VMS Software, Inc and Hewlett-Packard Development Company, L.P.
SYSTEM@x86vms$ cc /version
VSI C x86-64 X7.4-843 (GEM 50XB9) on OpenVMS x86_64 V9.2-1

Try to build with COMPILE.COM :

SYSTEM@x86vms$ @compile

MMK Bootstrap Procedure

$! If you have the SDL compiler installed, then feel free to uncomment the
$! lines below. However, it is not absolutely necessary as the resulting
$! header files are distributed with the source kit.
$!
MESSAGE/NOOBJECT/SDL=MMK_MSG.SDL MMK_MSG.MSG
SDL/VAX/LANGUAGE=CC=ETC_DIR:MMK_MSG.H MMK_MSG.SDL
$ LIBRARY/CREATE BIN_DIR:MMK.OLB
$ CC/NOLIST/OBJECT=BIN_DIR:MMK.OBJ MMK.C
$ LIBRARY/REPLACE BIN_DIR:MMK.OLB BIN_DIR:MMK.OBJ
$ CC/NOLIST/OBJECT=BIN_DIR:FILEIO.OBJ FILEIO.C
$ LIBRARY/REPLACE BIN_DIR:MMK.OLB BIN_DIR:FILEIO.OBJ
$ CC/NOLIST/OBJECT=BIN_DIR:MEM.OBJ MEM.C
$ LIBRARY/REPLACE BIN_DIR:MMK.OLB BIN_DIR:MEM.OBJ
$ CC/NOLIST/OBJECT=BIN_DIR:GET_RDT.OBJ GET_RDT.C
$ LIBRARY/REPLACE BIN_DIR:MMK.OLB BIN_DIR:GET_RDT.OBJ
$ CC/NOLIST/OBJECT=BIN_DIR:SP_MGR.OBJ SP_MGR.C
$ LIBRARY/REPLACE BIN_DIR:MMK.OLB BIN_DIR:SP_MGR.OBJ
%LIBRAR-E-DUPGLOBAL, global symbol exit_handler from file DKA0:[WORK.mmk.BIN-X86_64]SP_MGR.OBJ;1 already in library DKA0:[WORK.mmk.B
IN-X86_64]MMK.OLB;1
$BAIL_OUT:
$ EXIT 1.or.(0*F$VERIFY(0))

This is one of the simplest solution avoiding duplicate names in the library.
It builds and works well on OpenVMS x86_64