[GRASS5] r.flow: strange compile error M_PI

[Markus Neteler]

Hi,

after an update of the Linux OS, I am no longer able to
compile r.flow:

thuille:r.flow[281.25] make
gcc -I/hardmnt/thuille0/ssi/software/cvsgrass61/include -I/hardmnt/thuille0/ssi/software/cvsgrass61/dist.i686-pc-linux-gnu/include/grass  -g -Wall -ansi  -Wall -Wconversion -Wno-implicit-int   -I/usr/local/include     -DPACKAGE=\""grassmods"\"  -I/hardmnt/thuille0/ssi/software/cvsgrass61/include -I/hardmnt/thuille0/ssi/software/cvsgrass61/dist.i686-pc-linux-gnu/include/grass \
        -o OBJ.i686-pc-linux-gnu/aspect.o -c aspect.c
aspect.c: In function `aspect_fly':
aspect.c:50: error: `M_PI' undeclared (first use in this function)
aspect.c:50: error: (Each undeclared identifier is reported only once
aspect.c:50: error: for each function it appears in.)
make: *** [OBJ.i686-pc-linux-gnu/aspect.o] Error 1

thuille:r.flow[282.26] cat /etc/issue
Red Hat Enterprise Linux WS release 4 (Nahant Update 1)
Kernel \r on an \m

It should be in math.h, right?
How to debug such things?

Mysterious (for me),

 Markus