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.
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.