pgxs question - linking c-functions to external libraries

Hi

I have written a number of C-Function which compile and link against the GNU
Scientific Library libraries 'libgsl' and 'libgslcblas' when using a hand
written Makefile, but I would like to use PGXS.

My functions compile fine using PGXS, but don't link against the GSL libraries.
How do I specify the GSL libraries as external libraries to link against in the
PGXS Makfile? I have tried the following which don't work:

PG_CPPFLAGS = -lgsl -lgslcblas
PG_LIBS = -lgsl -gslcblas
SHLIB_LINK = -lgsl -gslcblas

My PGXS makefile is:

MODULES pg_gsl
PGXS := $(shell $(PG_CONFIG) --pgxs)
include $(PGXS)

Many thanks
John