compile error pan-0.14.2 lvalue required

I tried to install pan-0.14.2, but got compilation errors in the make
process.
The listing out of "./INSTALL" I got:

Making all in base
make[1]: Entering directory `/home/julien/local/pan-0.14.2/pan/base'
gcc -DHAVE_CONFIG_H -I. -I. -I../.. -I../.. -I/usr/local/include/pan
-DGNOMELOCALEDIR=\""/usr/local/share/locale"\"  -I/usr/include/gtk-2.0
-I/usr/lib/gtk-2.0/include -I/usr/include/atk-1.0 -I/usr/include/cairo
-I/usr/include/pango-1.0 -I/usr/include/glib-2.0
-I/usr/lib/glib-2.0/include -I/usr/include/freetype2
-I/usr/include/libpng12   -pthread -I/usr/include/glib-2.0
-I/usr/lib/glib-2.0/include   -I/usr/include/libxml2        -g -O2 -I.
-c msort.c
msort.c: In function 'msort_with_tmp':
msort.c:68: error: lvalue required as increment operand
msort.c:69: error: lvalue required as increment operand
msort.c:74: error: lvalue required as increment operand
msort.c:75: error: lvalue required as increment operand
make[1]: *** [msort.o] Error 1
make[1]: Leaving directory `/home/julien/local/pan-0.14.2/pan/base'
make: *** [all-recursive] Error 1

the corresponding listing (including some line numbers) of the
corresponding source-lines of pan-0.14.2/pan/base (61 till 77)

61     if (s == opsiz && (b1 - (char *) 0) % opsiz == 0)
              /* operating on aligned words.  Use direct word stores. */
                while (n1 > 0 && n2 > 0)
64              {
                        if ((*cmp) (b1, b2) <= 0)
                        {
                                --n1;
68                              *((unsigned long int *) tmp)++ =
69                                      *((unsigned long int *) b1)++;
                        }
                        else
                        {
                                --n2;
74                              *((unsigned long int *) tmp)++ =
75                                      *((unsigned long int *) b2)++;
                        }

I'm surprised by these errors on a program that's around since June. I'm
afraid it will be my mistake, but I don't know where to search. Any help
will be greatly apreciated.
Thanks
-- 
Julien Michielsen
julien_at_michkloo.xs4all.nl