GSL version issue on MacOS

I have an issue where ./configure fails on my MacOS laptop with a GSL version error, while the GSL verison is high enough ...:

$ ./configure --prefix=/Users/vivien/conda/ligo-py36/
checking for gcc... x86_64-apple-darwin13.4.0-clang
checking whether the C compiler works... yes
checking for C compiler default output file name... a.out
[...]
checking for GSL... yes
checking for gsl_version in -lgsl... yes
checking for GSL version >= 1.15... configure: error: could not find required version of GSL

This is using environment-py36.yml. config.log is attached. But, in this ligo-py36 environment:

$ pkg-config --libs gsl
-L/Users/vivien/miniconda3/envs/ligo-py36/lib -lgsl -lgslcblas -lcblas -lm
$ which gsl-config
/Users/vivien/miniconda3/envs/ligo-py36/bin/gsl-config
$ gsl-config --version
2.4

And 2.4 >= 1.15 (I checked on google.com). There could be something fishy with my installation, but I hoped being inside the conda environment would isolate me from the mess that is /opt on my laptop.