finding GSL when in non-standard path

[hovr2pi]

If you are using GSL installed in a non-standard path but DO NOT have the gsl-config binary in your path (that is you do not have $GSL_DIR/bin in your path), configure will find the headers but can not find gsl-config and will therefore not add GSL to the build.

I presume the configure continues and completes because GSL is optional.

[pbauman]

Thanks. We need to add the GSL_DIR bits in the AC_CHECK_PROG for gsl-config.