how to do it on GNU/Linux:
@smallexample
-gcc -shared -o libguile-bessel.so -fPIC bessel.c
+gcc `pkg-config --cflags guile-@value{EFFECTIVE-VERSION}` \
+ -shared -o libguile-bessel.so -fPIC bessel.c
@end smallexample
For creating shared libraries portably, we recommend the use of GNU